Optical power splitter with electrically-controlled switching structures

ABSTRACT

A new class of optical energy transfer devices and energy guiding devices uses an electric field to control energy propagation using a class of poled structures in solid material. The poled structures, which may form gratings in thin film or bulk configurations, may be combined with waveguide structures. Electric fields applied to the poled structures control routing of optical energy. Devices include splitters, parallel and Y couplers, mode converters and energy leakage attenuators.

BACKGROUND OF THE INVENTION

This invention relates to devices, particularly optical devices, forcontrolling propagation of energy, particularly optical beams, usingelectric field control. In particular, the invention relates to deviceswith poled structures, including periodically poled structures, andelectrodes which permit controlled propagation of optical energy in thepresence of controlled electric fields applied between electrodes.

More particularly, the invention relates to a new class of switchableenergy conversion devices, energy guiding devices, filters, and bulkenergy transfer devices based on the use of poled structures in solidstate material. In some applications, the poled structures can beswitched electrically to control optical or even acoustic energy. Apoled switch is especially applicable to the fields of laser control,communications, flat panel displays, scanning devices and recording andreproduction devices.

Interactions with energy beams such as optical or acoustic beams can becontrolled by means of applied electric fields in electro-optic (EO) orpiezoelectric materials. An electrically controlled spatial pattern ofbeam interaction is desired in a whole class of switched or modulateddevices. Patterned responses can be achieved in uniform substrates usingthe electro-optic or piezoelectric effect by patterning the electricfield. However, Maxwell's equations for the electric field prevent sharpfield variations from extending over a large range. Some materials canbe poled, which means their electro-optical and/or piezoelectricresponse can be oriented in response to some outside influence. In thesematerials, is possible to create sharp spatial variations in EOcoefficient over potentially large ranges. By combining slowly varyingelectric fields with sharply varying (poled) material, new types ofpatterned structures can be fabricated and used.

Polable EO materials have an additional degree of freedom which must becontrolled, as compared to fixed EO crystals. Usually, the substratemust be poled into a uniformly aligned state before any macroscopic EOresponse can be observed. Uniformly poled substrates have beenfabricated both from base materials where the molecules initially haveno order, and from base materials where the molecules spontaneouslyalign with each other locally, but only within randomly orientedmicroscopic domains. An example of the first type of material is thenonlinear polymer. Examples of the second type of material are sinteredpiezoelectric materials such as lead zirconate titanate (PZT), liquidcrystals, and crystalline ferroelectric materials such as lithiumniobate (LiNbO₃). Nonlinear polymer poling is described in ♦ E. VanTomme, P. P. Van Daele, R. G. Baets, P. E. Lagasse, "Integrated opticdevices based on nonlinear optical polymers", IEEE JQE 27 778, 1991. PZTpoling is described for example in ♦ U.S. Pat. No. 4,410,823, October1983, Miller et al, "Surface acoustic wave device employing reflectors".(Liquid crystal poling is described in standard references, such as S.Chandrasekhar, Liquid Crystals, Second Edition (1992), CambridgeUniversity Press, Cambridge.) Ferroelectric crystal poling is describedin ♦ U.S. Pat. No. 5,036,220 July 1991, Byer et al., "Nonlinear opticalradiation generator and method of controlling regions of ferroelectricpolarization domains in solid state bodies".

Examples of poled EO devices include:

♦ the beam diffractor in a polymer layer with interdigitated electrodesof S. Ura, R. Ohyama, T. Suhara, and H. Nishihara, "Electro-opticfunctional waveguide using new polymer p-NAn-PVA for integrated photonicdevices," Jpn. J. Appl. Phys., 31, 1378 (1992) [UOS92];

♦ the beam modulator in a polymer layer with planar electrodes of U.S.Pat. No. 5,157,541 October 1992, Schildkraut et al. "Optical article forreflection modulation";

♦ the total internal reflection beam reflector in a lithium niobatewaveguide with an electrode pair of H. Naitoh, K. Muto, T. Nakayama,"Mirror-type optical branch and switch", Appl. Opt. 17, 101-104 (1978);

♦ the 2×2 waveguide switch in lithium niobate with two electrodes of M.Papuchon, Am. Roy, "Electrically active optical bifurcation: BOA", Appl.Phys. Lett. 31, 266-267 (1977); and

♦ the wye junction beam router in a lithium niobate waveguide with throeelectrodes of H. Sasaki and I. Anderson, "Theoretical and experimentalstudies on active y-junctions in optical waveguides", IEEE Journ. Quant.Elect., QE14, 883-892 (1978).

These devices use uniformly poled material with varied electrode andoptical structures. Many of the advantages of patterned poled deviceshave not been recognized. For example, in the book by ♦ H. Nishihara, M.Haruna, T. Suhara, Optical Integrated Circuits, McGraw-Hill, New York(1989) [NHS89], many electro-optical devices activated by variouselectrode patterns are described, but all of these devices arefabricated on a uniformly poled substrate. The same is true of anotherreview article, ♦ T. Suhara and H. Nishihara, "Integrated opticscomponents and devices using periodic structures," IEEE J. QuantumElectron., QE-22, 845, (1986) [TH86], which describes the generalcharacteristics of grating coupled devices without recognizing theadvantages of a poled grating as opposed to an electrode grating.

In selected instances in the literature, certain advantages of patternedpoled substrates have been pointed out.

♦ A surface acoustic wave reflector with an array of domain reversals ina piezoelectric ceramic (but no electrodes) is described in U.S. Pat.No. 4,410,823, Miller et al.;

♦ A beam steerer with triangular domain reversed regions in LiTaO₃ isdescribed in Q. Chen, Y. Chiu, D. N. Lambeth, T. E. Schlesinger, D. D.Stancil, "Thin film electro-optic beam deflector using domain reversalin LiTaO₃ ", CTuN63, CLEO'93 Conference Proceedings, pp 196 et. seq.,Optical Society of America.

♦ A Mach-Zehnder modulator with domain reversals to compensate phasedifferences between microwave and optical beams is described in U.S.Pat. No. 5,278,924, January 1994, Schaffner, "Periodic domain reversalelectro-optic modulator".

♦ A Mach-Zehnder electric field sensor with one domain reversed regionin an electro-optic substrate is described in U.S. Pat. No. 5,267,336,November 1993, Sriram et al., "Electro-optical sensor for detectingelectric fields".

Use of patterned poled structures offers efficiency advantages in beamcontrol (including generation, modulation, redirection, focussing,filtration, conversion, analysis, detection, and isolation) withapplications in laser control; communications; data storage; anddisplay. What is needed in these areas are adjustable methods for beamcontrol with high efficiency. Due to the sharp domain transitions,higher efficiency devices can generally be obtained using pattern poledsubstrates to create the high frequency variations; the electrodes areneeded to excite the patterned poled substrate, not to create the highfrequency variations.

The poling process in polymers is quite different from that of crystals,and results in poorly defined domain boundaries. In crystals, there area discrete number of (usually two) poling directions which are stable,and poling a local region consists of flipping atoms between thesealternative states. Poled regions are fully aligned, and sharpboundaries exist between oppositely aligned domains. In poled polymers,any molecule can be oriented in any direction regardless of the polingdirection. The poling process produces only an average component ofalignment within a random distribution of individual molecules. Inpolymers, the poling (and the related EO coefficients) therefore have acontinuous variation in strength and orientation. The sharp domainboundaries obtained in crystals are absent. This has a profoundinfluence on the efficiency of certain types of poled device inpolymers. Since the poling strength and direction in polymers followsthe strength and direction of the local applied electric field, it isnot possible to obtain poling features with spatial dimensions anysharper than permitted by Maxwell's equations. In polymers, there isvery little advantage to be obtained from spatially patterning the poledregions instead of the electrodes.

In devices based on optical polymers, poling is required to create anelectro-optical response. The poling is done by applying a voltage toelectrodes fabricated on the device (in the presence of heat). Theentire polymer film may be poled with a uniform electrode, after whichthe electrodes are spatially patterned for the desired functionality.The EO performance of the device will not change much if the poling isaccomplished with the patterned electrodes, since the active regionwithin reach of the electric field is still poled almost as well. Thechoice of whether to pole the whole layer or just the region under theelectrodes is mainly by convenience in fabrication. Examples of polymerEO devices where the poling is spatially patterned outside the activeregion of the device are ♦ the switched waveguides of U.S. Pat. No.4,867,516, September 1989, Baken et al., "Electro-optically inducedoptical waveguide, and active devices comprising such a waveguide", and♦ U.S. Pat. No. 5,103,492, April 1992, Ticknor et al., "Electro-opticchannel switch". None of these devices have the electrodes traversemultiple boundaries of a patterned poled structure.

The poling process also changes the index of refraction ellipsoid inpolymers. This fact has some desirable consequences, such as makingpossible waveguides fabricated by poling a stripe of polable polymer asdescribed in ♦ J. I. Thackara, G. F. Lipscomb, M. A. Stiller, A. J.Ticknor, and R. Lytel, "Poled electro-optic waveguide formation inthin-film organic media," Appl. Phys. Lett., 52, 1031 (1988) [TLS88] andin ♦ U.S. Pat. No. 5,006,285, April 1991, and U.S. Pat. No. 5,007,696.April 1991, Thackara et al. "Electro-optic channel waveguide". However,it leaves a problem in that poled polymer boundaries are lossy in theirunexcited state (they scatter, diffract and refract). Devices in which alight beam crosses poled polymer boundaries have the problem thatalthough transparency may be achieved, the poled polymer must beactivated electrically to produce a uniform index of refraction. Poledcrystalline devices do not have this problem because poling does notchange their index of refraction.

A solution to the problem of lack of transverse spatial definition inpoled polymers was proposed in ♦ U.S. Pat. No. 5,016,959 May 1991,Diemeer, "Electro-optical component and method for making the same", whodescribe a total internal reflection (TIR) waveguide switch in which theentire polymer film is poled, but the electro-optic coefficient ofselected regions is destroyed by irradiation, creating unpoled regionswith sharp spatial boundaries. While the underlying molecules in theseunpoled irradiated regions remain aligned, they no longer have anyelectro-optic response. This approach is useful in creating sharppoled-unpoled domain boundaries in polymer films. It has thedisadvantage that it cannot produce reverse poled domains so itsefficiency is considerably reduced compared to the equivalent crystalpoling technique.

In nonlinear frequency conversion devices, domains of different polarityare typically periodically poled into a nonlinear optic material, butnot excited by an electric field. The poled structure periodicallychanges along the axis of the beam to allow net energy conversiondespite a phase difference that accumulates between the two beams. Thisprocess is known as quasi-phasematching, and has been demonstrated inferroelectrics [U.S. Pat. No. 5,036,220, Byer et al.] such as lithiumniobate, KTP, and lithium tantalate, as well as in polymers, asdescribed in ♦ U.S. Pat. No. 4,865,406 September 1989, Khanarian et al,"Frequency doubling polymeric waveguide". Electrodes are not typicallyused in these devices, since the phasematching occurs in the absence ofan electric field. Generalized frequency conversion in polymers isdescribed in ♦ U.S. Pat. No. 5,061,028 October 1991, Khanarian et al,"Polymeric waveguides with bidirectional poling for radiation phasematching", as well as TE-TM modulation. Khanarian et al. used patternedelectrodes in both patents to pole the polymer film; the attendant lossin sharpness of the spatial pattern becomes a severe problem where morecomplex electrode structures are needed such as in the latter patent.

Devices are known employing periodic structures which use electricfields to control gratings in order to control propagating fields. Adiffraction grating modulator is shown in ♦ U.S. Pat. No. 4,006,963,February 1977, Baues et al. "Controllable, electro-optical gratingcoupler". This structure is fabricated by removing material periodicallyin an electro-optic substrate to form a permanent grating. By excitingthe substrate electro-optically, the fixed index grating has a greateror lesser effect, producing some tuning. This structure does not containpoled regions. The drawbacks of the Baues structure are the same as forthe polymer film: the grating cannot be made transparent without theapplication of a very strong field.

The current technology for an EO switchable grating is shown in FIG. 1(Prior Art). In this structure, periodically patterned electrodes serveas the elements that define the grating. The underlying material doesnot have a patterned poled structure, as hereinafter explained. An inputbeam 12 is coupled into a electro-optically active material 2 whichcontains an electrically controllable grating 6. When the voltage source10 to the grating electrodes is off, the input beam continues topropagate through the material to form the output beam 16. When thegrating-controlling voltage source is switched on, an index modulationgrating is produced in the material, and a portion of the input beam iscoupled into a reflected output beam 14. The material has anelectro-optically active poled region 4 with a single domain, with thesame polarity throughout the poled structure. A first electrode 6 isinterdigitated with a second electrode 7 on a common surface 18 of thesubstrate. When a voltage is applied between the electrodes, thevertical component of electric field along the path of the beam 12alternately has opposite sign, creating alternate positive and negativeindex changes to form a grating. The strength of the grating iscontrolled by the voltage source connected between the two electrodes bytwo conductors 8.

A second general problem with the existing art of EO and piezoelectricdevices using uniform substrates and patterned electrodes is that thepattern of the excited electric field decays rapidly with distance awayfrom the electrodes. The pattern is essentially washed out at a distancefrom the electrodes equal to the pattern feature size. This problem isaggravated in the case of a grating because of the very small featuresize. Prior art gratings formed by interdigitated electrodes produce amodulated effect only in a shallow surface layer. EO structures interactweakly with waveguides whose dimension is larger than the feature size.While longer grating periods may be used in higher order interactiondevices, the lack of sharp definition described above again seriouslylimits efficiency. The minimum grating period for efficient interactionwith current technology is about 10 microns. What is needed is a way tomaintain the efficiency of EO devices based on small structures, despitea high aspect ratio (i.e. the ratio of the width of the optical beam tothe feature size). Switchable patterned structures are needed whichpersist throughout the width of waveguides and even large unguidedbeams.

In bulk material, gratings may be formed by holographic exposure andacoustic excitation. Holographic exposure is very difficult, and storagematerials such as SBN are not yet developed to a commercial state.Acoustic excitation is very expensive to implement and to power, andrequires additional components such as soft mounts and impedance matcheddamping structures. Other methods form surface gratings, includingdeposition techniques, material removal techniques and materialmodification techniques (such as indiffusion, outdiffusion, and ionexchange). What is needed is an approach capable of a large enoughaspect ratio to produce bulk interaction structures, preferably withfeature control at an accessible surface.

While the EO material can in principle be any electro-optically activematerial, liquid crystals are a special case and have limitedapplicability. A light modulator based on diffraction from an adjustablepattern of aligned liquid crystal domains is described in ♦ U.S. Pat.No. 5,182,665, January 1993, O'Callaghan et al., "Diffractive lightmodulator". A light modulator based on total internal reflectionmodulated by liquid crystal domain formation is described in ♦ U.S. Pat.No. 4,813,771 March 1989, Handschy et al., "Electro-optic switchingdevices using ferroelectric liquid crystals". In all of these devices,the domains must physically appear or disappear to produce the desiredeffect. The orientation of the molecules in the liquid crystal devicechanges in response to an applied field, producing a patterned structurewhich interacts with light. However, liquid crystals have importantdrawbacks. They are of course liquid and more difficult to package, andthey have a limited temperature range and more complex fabricationprocess than solid state devices. High aspect ratio structures cannot bemade because of the decay of the exciting field pattern with distance.The molecular orientation relaxes as soon as the field is turned off,and re-establishing the pattern takes a long time, so fast switching isnot possible.

The structures which switch light from waveguide to waveguide in theprior art have a high insertion loss or large channel spacing whichrender them unsuitable for large routing structures. A large switchingstructure must have switching elements with insertion loss low enough topermit light to propagate through the structure. If a waveguide has 100switches, for example, the switches must have less than about 0.03 dBinsertion loss. In the prior art this is not possible. R. A. Becker andW.S.C. Chang, "Electro-optical switching in thin film waveguides for acomputer communications bus", Appl. Opt. 18, 3296 (1979), demonstrate amultimode crossing waveguide array structure coupled via interdigitatedelectro-optic grating switches. This switch has an inherently highinsertion loss (0.4 dB) and poor switching efficiency (≈10%). U.S. Pat.No. 5,040,864, August 1991, J. H. Hong, "Optical Crosspoint SwitchModule", discloses a planar waveguide structure which may in principlehave a low insertion loss, but which requires very large crossingjunctions for efficient switching, and is therefore incapable ofproducing a high density switching array

In summary, the prior art has shortcomings in several areas: 1) largeaspect ratios of controllable patterns are needed for efficientinteraction with bulk waves or small patterns; 2) sharp domaintransitions are needed for efficiency in higher order interactions; 3)transparency of domain structures is needed at zero applied field forproper unpowered operation; and 4) low insertion loss is required forarrays of switches. Poled structures contained in the above and otherstructures have not been fully utilized heretofore to realize practicaldevices.

SUMMARY OF THE INVENTION

According to the invention, a new class of optical energy transferdevices and energy guiding devices uses an electric field to controlenergy propagation using a class of poled structures in solid material.The poled structures, which may form gratings in thin film or bulkconfigurations, may be combined with waveguide structures. Electricfields applied to the poled structures control routing of opticalenergy. Devices include splitters, parallel and Y couplers, modeconverters and energy leakage attenuators.

The invention will be better understood upon reference to the followingdetailed description in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a modulator with interdigitated electrodes, according to theprior art.

FIG. 2 is a generalized embodiment of the switched grating forinteracting with bulk optical beams, according to the invention.

FIG. 3 is an embodiment of a waveguide retroreflector using the switchedgrating.

FIG. 4 is an embodiment of an electrode configuration for theretroreflecting device with three electrodes disposed on the same faceof the crystal.

FIG. 5 is an embodiment of an electrode configuration for the samedevice, in which two electrodes are disposed on the same face of thecrystal.

FIG. 6 is an embodiment of an electrode configuration for the device, inwhich three electrodes with tapered separation are disposed on the sameface of the crystal.

FIG. 7 is a tee embodiment of a poled crossing waveguide coupler.

FIG. 8 is an x embodiment of a poled crossing waveguide coupler.

FIG. 9 is an embodiment of a poled waveguide output coupler, with outputout of the plane of the waveguide.

FIG. 10 is an embodiment of a parallel waveguide poled directionalcoupler.

FIG. 11 is a top view schematic diagram of the an x crossing waveguidecoupler with illustrations of alternative input and output modeprofiles.

FIG. 12 is an embodiment of an x crossing waveguide coupler with taperedcoupling region geometry excited with a tapered electrode gap.

FIG. 13 is an embodiment of an x crossing waveguide coupler withgeneralized coupling region geometry and electrode pattern.

FIG. 14 is a bulk optics embodiment of a tunable-frequency poledelectro-optic retroreflector.

FIG. 15 is a waveguide embodiment of a tunable-frequency poledelectro-optic retroreflector.

FIG. 16 is a bulk optics embodiment of a tunable-frequency electro-opticretroreflector with electro-optic cladding and independent excitation ofpoled grating and cladding.

FIG. 17 is a waveguide embodiment of a multiple frequency poledelectro-optic retroreflector.

FIG. 18 is an illustration of a phase shifted poled grating.

FIG. 19 is an embodiment of a multiple period grating reflector.

FIG. 20 is an illustration of the frequency response curves of twodevices with multiple periodicity and different free spectral range.

FIG. 21 is an embodiment of a twin grating tunable reflector.

FIG. 22 is a schematic illustration of an integrated etalon consistingof twin gratings with adjustable optical path length.

FIG. 23 is an embodiment of a dual grating switchable wye junction withphase shifter.

FIG. 24 is an embodiment of a poled waveguide mode converter.

FIG. 25 is an embodiment of a waveguide router using the waveguide modeconverter.

FIG. 26 is an embodiment of a switchable parallel waveguide resonator.

FIG. 27 is an embodiment of a three-arm waveguide etalon.

FIG. 28 is an embodiment of a ring waveguide etalon.

FIG. 29A is an embodiment of a modulator/attenuator with controllablepoled mid-structure.

FIG. 29B is an embodiment of an adjustable lens structure.

FIG. 30 is an embodiment of a poled total internal reflecting (TIR)waveguide switch with switched poled waveguide stub.

FIG. 31 is an embodiment of a dual TIR waveguide switch.

FIG. 32 is an embodiment of a TIR electrically switched beam directorwith switched unpoled waveguide stub.

FIG. 33 is an embodiment of a two position poled waveguide routerwithout TIR.

FIG. 34 is an embodiment of an array of poled TIR switches with a 50%switch packing density.

FIG. 35 is an embodiment of an array of poled TIR switches with a 100%switch density.

FIG. 36 is an embodiment of a dual waveguide structure for high densitypacking architectures with permanent turning mirror and asymmetric losscrossing region.

FIG. 37 is an embodiment of a switched waveguide array with TIRswitches.

FIG. 38 is an embodiment of a switched waveguide array with gratingswitches.

FIG. 39A is an embodiment of an m×m communications switch array withsystem control lines.

FIG. 39B is an embodiment of a 3×3 switch array with WDM capability.

FIG. 40 is an embodiment of a two dimensional switching array with pixelelements.

FIG. 41 is an embodiment of a one dimensional switching array with pixelelements coupled to data tracks.

FIG. 42 is an embodiment of a switchable spectrum analyzer usingselectable grating reflector sections and a detector array.

FIG. 43 is an illustration of a poled acoustic multilayerinterferometric structure.

FIG. 44 is an illustration of a poled acoustic transducer.

FIG. 45 is an embodiment of a tuned coherent detector of multi-frequencylight waves.

FIG. 46 is an embodiment of a low loss switchable waveguide splitterusing a single poled region.

FIG. 47 is an embodiment of a low loss switchable waveguide splitterusing multiple poled regions.

FIG. 48 is an illustration of the key design elements for a 1×3waveguide splitter.

FIG. 49 is a multiple layer stack of active waveguide devices shown asan adjustable phased array modulator.

FIG. 50 is an embodiment of an adjustable waveguide attenuator of theprior art.

FIG. 51 is an embodiment of a multiple poled segment adjustablewaveguide attenuator.

FIG. 52 is an embodiment of a structure with widened bandwidth using anangle-broadened poled grating.

FIG. 53 is an embodiment of a structure with widened bandwidth using acurved waveguide.

FIG. 54 is an embodiment of an electrically controllable poled lens.

DESCRIPTION OF SPECIFIC EMBODIMENTS

This invention as claimed relates specifically to FIGS. 7, 8, 25, 46, 47and 51. Other figures generally relate to the claimed invention.Referring to FIG. 2, there is shown a generalized embodiment of a device11 of the present invention, which is a patterned poled dielectricdevice. Essentially, this device is an electrically-controllable stackeddielectric optical energy redirector, or more succinctly, anelectrically-switchable mirror. In a preferred embodiment, the inventionis a bulk optical reflector in a ferroelectric crystal 20 of lithiumniobate. The electrically-controlled switching element is a poledgrating 22, which consists of alternating poled domains of two types 36and 38.

A domain, which may be of any shape or size, is a physical region withinwhich certain material properties are approximately constant. A poleddomain is a region in a material in which the molecular groups have adirectionality and these groups are substantially aligned (or arepartially aligned) in, or near, a direction called the poling direction.There are many types of domains including domains of aligned atomicstructures in different directions, domains of aligned molecules oratomic structures with various modified parameters such as the nonlinearactivity or the electro-optic coefficient, domains of atomic structureswith no preferred direction, domains defined by regions activated bydifferent electrodes, poled regions in which the poling direction variessystematically across the region such as occurs in the case of polymersand fused silica poled with localized electrodes, domains of randomlyoriented molecules, and by extension, a random domain structure: domainsof sub-domains which are randomly poled within the domain. A poledstructure is a set of individual domains. A patterned poled region is aregion in a material in which the domains within the region have beenpoled according to a spatial pattern, with more than one domain type.There may be a systematic offset between the poled pattern and theimposed pattern used during the poling process, depending on the natureof this process. The boundaries of the pattern may also be somewhatirregular and not follow the imposed pattern perfectly, particularly ifthe poling process is not under complete control. The device isdescribed as a patterned poled dielectric because an electric field isapplied in controlling the device, so the material must be a dielectricin order to withstand the required field without damage. Typically, thepoling process is also accomplished using an electric field, which thematerial must also withstand. In general, we mean by dielectric thecapability of the material to withstand the minimum electric fieldsneeded for the application.

In operation, an optical input beam 40 is incident on and through thecrystal, along an optical axis. The optical axis is normal to the phasefront of the beam and is defined by the mean location of the propagatingbeam across its intensity profile at the phase front. The optical axisis straight in a uniform material, but may bend in several situationsincluding curved waveguides, nonuniform media, and in reflective ordiffractive structures. The input beam 40 preferably has a sufficientlysmall spot size 21 throughout the crystal length so that it is notapertured by the crystal, causing undesirable power loss and modeconversion. In a bulk-interaction device such as is shown in FIG. 2, thedomains 36 and 38 must penetrate a sufficient distance through thesubstrate 20 so that they overlap at least a portion of the input beam40. The grating 22 lies transverse of the input beam 40. This means theplanes 34 of the grating 22 are transverse of the axis of the input beam40. For two lines (or a line and a plane, or two planes) to betransverse of each other we mean that they are not parallel. Since thegrating is transverse of the beam 40, the beam passes through at least aportion of the structure of the grating 22.

The optical beam 40 is derived from an optical frequency source (notshown) and has a wavelength such that the beam is not substantiallyabsorbed in the crystal, and such that the photorefractive effect doesnot distort the beam significantly. The optical frequency source meansmay include one or more optical exciters capable of supplying sufficientbrightness within the wavelength acceptance of the grating reflector 22to produce a useful switched output beam 44. The output beam may becoupled to other elements on the same substrate, or it may be coupled toexternal devices, in which case the output surface through which beam 44emerges is preferably antireflection coated. The antireflection coatingmay be a multilayer dielectric coating, a single quarter wave layer of amaterial with almost the appropriate index of refraction, or a sol-gelcoating. The exciter may be any light source including a laser, a lightemitting diode, an arc lamp, a discharge, or even a filament, providedthat the desired spectral brightness is achieved. The desired spectralbrightness may be supplied directly from one or more exciters,indirectly from one or more frequency converted (doubled, mixed, orparametrically amplified) excites, or in combination with several of theabove alternatives. Absorption effects will limit the wavelength to therange from about 400 to 4000 nm. The effect of the photorefractivephenomenon varies with the configuration, the wavelength, dopants, andthe poling structure, and we assume here that it has been brought undercontrol so that any beam distortion remains within acceptable limits.

The grating 22 is formed or defined by the boundaries 34 betweenalternating domains of two different types. The first type of domain 36has a different electro-optic (E-O) coefficient than the second type ofdomain 38, so that a uniform electric field applied between theelectrodes 24 and 26 results in different changes in the index ofrefraction in the two types of domains. Because the index of refractionchanges the phase velocity of the wave, there is an impedance mismatchbetween the regions of different index or phase velocity. It isadvantageous to accomplish such an index change with material in whichthe regions 36 have a reverse sense relative to the poling direction ofthe other domain type 38 and the original wafer 20, as shown by thepoling sense arrows 39, 41. By reverse sense we mean the polingdirection is opposite to some reference direction. (An alternativerealization of the field controllable grating is in an irradiated maskedpolymer film which has its E-O coefficient destroyed inside or outsidethe regions 36.) A uniform electric field applied to the structure 22produces a modulated index of refraction. The pattern of indexmodulation adds to the pre-existing index of refraction distribution;the simplest configuration has no index modulation in the absence of theapplied electric field, and develops an index grating linearly inresponse to the applied field. A period 48 for the grating 22 is thedistance between two domain boundaries entirely including a regioncorresponding to each domain type.

An alternative realization of the index of refraction grating isobtained by applying a strain field to the poled regions. Thephotoelastic response of the material produces different index ofrefraction changes in the different poled regions. The strain field maybe applied permanently by, for example, laying down a film on top of thesubstrate at a high temperature and then cooling to room temperature. Aconcentration of strain may be achieved by etching away a stripe of thefilm, for example.

The poled elements 36 and 38 alternate across the grating 22 with nospace between them. If additional domain types are available, morecomplicated patterns of alternation are possible with domains separatedby variable distances of the different domain types. For someapplications, the grating 22 is a uniformly periodic grating as shown inFIG. 2 so that the domain types contained in one period along the lengthof the grating 22 are reproduced in the other periods. For otherapplications, it is advantageous to modify the period to obtainadvantages such as multiple spectral peaks or a broader spectralbandwidth. By grating we mean an array of distinguishable structures,including all possible variations of geometry and periodicity.

A periodic index grating is capable of supplying virtual photons in aninteraction between optical beams. This means the grating structure iscapable of supplying momentum, but not energy, to the interaction. Foran interaction to proceed, both energy and momentum must be conserved,and the grating is useful when a momentum increment is requited tosimultaneously satisfy the two conservation relations. The gratingperiodicity defines the momentum which is available to the interaction.The grating strength determines the "intensity" of the virtual photonbeam. The number of periods in the section of the grating traversed bythe optical beam determines the bandwidth of the virtual photon momentswhich are available. Because of the bandwidth limitation, theinteraction can only proceed within a specific range (or ranges) ofoptical frequencies. Grating devices are therefore inherently frequencyselective, and typically operate around a nominal wavelength.

For example, in a simple reflection process at an angle, as illustratedin FIG. 2, the photons of the input beam 40 have the same opticalfrequency as the photons of the output beams 44 and 42, so energyconservation is observed. However, the momentum of the photons in inputbeam 40 and diverted output beam 44 are not the same; for the reflectionprocess to occur, the change in momentum must be supplied by the grating22 as illustrated by the vector diagram 43 associated with FIG. 2. Thegrating 22 supplies a virtual (with momentum but no energy) photon tothe interaction to enable the conservation of momentum. The momentumvector associated with the i^(th) mode, ki=2πn_(i) /λ_(i), is equal tothe product of 2π times the effective index n_(i) for that mode dividedby the wavelength λ_(i) for that wave, and it points in the direction ofpropagation. The magnitude of the momentum vector is also called thepropagation constant. In the case of a single period grating, themomentum vector k_(g) =2π/Λ points perpendicular to the gratingsurfaces, and it can have any wavelength value Λ which is present in theFourier transform of the grating. The optical spacing (the width of thegrating lines and spaces) associated with the propagation constant kg ofa 50% duty cycle grating is therefore Λ/2. The frequency of interactionmay also be tuned by adjusting for example the index of refraction ofthe optical beams, or the grating period by thermal expansion or othermeans. Depending on how a given device is implemented, an indexstructure may have a spectrum of wavelengths and vector directions whichcan be contributed to the interaction. Also, multiple virtual photonsmay be contributed to an interaction in a so-called "higher order"grating interaction. A "higher order" grating is one which has a periodwhich is related to the required period for momentum conservation bydivision by an integer. The required momentum virtual photon is obtainedfrom the harmonics of the "higher order" grating. The condition thatmomentum be conserved by the process is commonly called the Braggcondition, so the gratings of this invention are Bragg gratings, and theincidence angle on the gratings is the Bragg angle for the in-band orresonant frequency component. This dual conservation of energy andmomentum is required for any energy beam interaction, whether the energybeam is optical, microwave, acoustic, or any other wavelike energy formconsisting of a time-variable energy field. Only the implementation ofthe grating may change, to produce an impedance modulation for thedifferent forms of energy so that the pattern of the structure cancouple with the wavelike energy form.

In FIG. 2, the index grating functions as a frequency-selective opticalenergy router or reflector. A beam of a characteristic frequency withinthe interaction bandwidth (capable of interacting with one or more ofthe virtual photons) is known as an in-band beam, while energy beams ofother frequencies are known as out-of-band beams. The grating 22 has afrequency bandwidth which corresponds to the full width at half maximumof the reflection efficiency of the grating as a function of opticalfrequency. When the index grating is present (the grating is "on"), abeam having an optical frequency within the bandwidth of the grating isreflected from the grating at the angle 46 around a normal 47 to thegrating structure. An out-of-band beam transmits through the crystalalong the same optical axis and in the same direction as the input beam,forming part of the transmitted output beam 42. An electric fieldapplied in the region including the grating controls the strength of theindex modulation (which can also be thought of as the intensity of thevirtual photons), adjusting the ratio of the power in the transmittedoutput beam 42 to that in the reflected output beam 44.

For a weak retro reflecting grating (which does not substantiallydeplete the input beam), the full width half maximum bandwidth Δλ isgiven by ##EQU1## where λ=vacuum wavelength of the input beam,

n=index of refraction of the beam, and

L=length of the grating.

For highly reflecting gratings, the effective length is smaller than thetotal length of the grating, increasing the bandwidth.

The two types of domains may exhibit an index difference before anelectric field is applied. In this case, a permanent index gratingaccompanies the poled switchable index grating. As the electric field isapplied, the net modulation in the index of refraction (the gratingstrength) may be increased or decreased, depending on the polarity. The"grating off" situation (index grating value near zero) is then achievedat a specific value of applied field. The grating can then be turned"on" by applying any other field strength. If the polarity of theapplied field is reversed, for example, an index grating is producedwith twice the strength of the original permanent grating.

The poled grating structure of our invention has two major advantagesover the prior art. First, the poled domain structures can have verysharp boundaries, providing a strong Fourier coefficient at virtualphoton moments which are multiples of the momentum corresponding to thebasic grating period. This is very useful in cases where it isimpractical to perform lithography with the required small feature size.Second, strong index modulation gratings can be made even if the opticalmode dimension is large compared to the grating period. This is notpossible in a uniformly poled substrate excited by patterned electrodes,because the electric field modulation decays exponentially with distanceaway from the plane of the electrode array, losing most of themodulation within a distance equal to the grating period. The polingprocess can create poled features with an extremely high aspect ratio,or the ratio of depth of the domain to its width. Using an electricfield poling technique, aspect ratios in excess of 250:1 have beenfabricated. Because we use essentially uniform electrodes, we get goodelectrostatic penetration; with deep domain walls, good modulation isavailable across the entire beam.

The grating may also be a two dimensional array of index changes, inwhich case the grating has periodicities in two dimensions. The virtualphoton contributed by the grating can then contribute momentum in twodimensions. This might be useful, for example, in an application withseveral output beams from a single grating.

In the preferred embodiment, the ferroelectric crystal is acommercially-available, z-cut, lithium niobate single-crystal wafer.Other cuts, including x-, y-, and angle-cuts can also be used, dependingon the poling method and the desired orientation of the poled domains.The fabrication steps include primarily poling and electrodefabrication. Prior to processing, the crystal is cleaned (for example byoxygen plasma ashing) to remove all hydrocarbons and other contaminantsremaining from the polishing and handling processes. To control thepoling, a mask and processing electrodes are used to create a pattern ofapplied electric field at the surface of and through the wafer, asdescribed in U.S. patent appl. Ser. No. 08/239,799 filed May 9, 1994.The poling pattern is adjusted to produce the poled domain inversion inregions 36 during the application of the poling field. In brief, asilica layer several microns thick is deposited on the +z surface 23 ofthe wafer 20. This film is thinned or removed over the regions 36 wheredomain inversion is desired, a liquid electrode or deposited metal filmis used to make a good equipotential surface over the patterned silica,and an electric field exceeding approximately 24 kV/mm is applied withthe +z surface 23 at a higher potential than the -z surface 25. Usingthis technique, ferroelectric crystals of lithium niobate have beenpoled to create patterns of two domain types which are of reversepolarity (domain inversion). The magnitude of the electro-opticcoefficient for the two types of domains is identical, although with areverse polarity.

In addition to the preferred technique, domain inversion has beenachieved in ferroelectrics using in-diffusion, ion-exchange, andalternate electric field poling techniques. Domain formation bythermally-enhanced in-diffusion has been demonstrated in lithiumniobate, using titanium. The triangular shape of the inverted regionlimits the interaction efficiency for small domain size, however, and isuseful mainly in waveguide devices with long periods. Patterned polingvia ion exchange has been demonstrated in KTP in a salt bath containingrubidium and barium ions, in which the potassium ions in the crystalwere exchanged for the rubidium ions. Electric field poling usingalternate techniques to the preferred one have also been demonstrated inboth lithium niobate and lithium tantalate. Potentially, all solidferroelectric materials, including KTP and barium titanate, can be poledby electric field domain-inversion techniques. (Solid means holding itsstructure for a certain period of time, such as cooled fluids, glasses,crosslinked polymers, etc.)

Gratings with different characteristics are generated by the differenttechniques. Electric field poling aligns the domains in the crystalwithout producing an intrinsic change in the index of refraction, whilethe ion-exchange and diffusion techniques do create a index change inthe poled regions. A permanent index grating accompanies the switchablepoled grating when these latter methods are used.

In general, there are two types of differing domains, at least the firsttype of which is poled. Although only two types of domains are required,more complex switchable grating structures can be fabricated withadditional types of domains. The second domain type may be reversepoled, unpoled, or poled at another angle, and it may be distinguishedby possessing a distinct electrical activity coefficient, (e.g. theelectro-optic or the piezo-optic coefficient). For example, it may insome applications be cost effective to fabricate the device from unpoledlithium niobate wafers, in which case the substrate wafer is comprisedof multiple randomly oriented domains. The poled domains will have auniform orientation while the orientation in the other domains will berandom. The performance of the device will be affected by the details ofthe random pattern, depending on the type of device. As another example,the second domains may be oriented perpendicular to the first or atanother angle, and the difference in the electrical response can stillproduce a useful electronically controlled structure. The poled domainsmay also be formed in a material which was previously unpoled andrandomly oriented on a molecular scale, such as in fused silica orpolymers. The poling process orients the structure of the material toform the first domain type, while the second domain type consists of theunpoled or randomly oriented regions in the material.

In an alternate technique, the poled structure can be formed byselectively changing or destroying the electrical activity coefficientin regions corresponding to the second domain type. The orientation ofthe atomic structures in these regions does not need to be altered: ifthe electrical activity is changed in the second domain region, thedomains are different. For example in nonlinear polymers, theelectro-optic coefficient may be disabled by irradiation, producingregions of electrical activity where the irradiation is masked off. Asimilar effect has been demonstrated in lithium niobate, where protonexchange destroys the nonlinear coefficient. Modification of theelectro-optic coefficient can also be achieved by optical radiation,electron bombardment, and/or ion bombardment in many other materials,including most nonlinear materials such as KTP and lithium tantalate.

In lithium niobate, an applied field E₃ along the z axis of the crystalinduces a change in the extraordinary index of refraction δn_(e) whichis given by ##EQU2## where r₃₃ is the appropriate electro-opticnonlinear optical coefficient. Because r₃₃ is the largest nonlinearconstant in lithium niobate, it is best to use the change in theextraordinary index in practical devices. (The nonlinear constant r₁₃which produces a change in the ordinary index of refraction due to anapplied E₃, is a factor of 3.6 smaller than r₃₃.) To use the change inthe extraordinary index, the light waves must be polarized along the zaxis of the material. In a z-cut crystal, this polarization is calledTM. (In TE polarization, the electric vector lies in the plane of thecrystal surface. The only other significant nonlinear coefficient isr₁₅, which couples TE and TM waves upon the application of an electricfield E₁ or E₂.)

Because the index change induced in the poled structures is quite small(with an applied field of 10 V/μm along the z axis of a lithium niobatesubstrate, the index change δn_(e) is only 1.6×10⁻³), the gratingreflector of FIG. 2 has a strong angular dependence. The Brewster anglefor a weak index change is 45°, so the gratings will totally transmitany TE polarized wave when the planes of the grating are disposed at andangle of 45° with respect to the phase front of the light beam. Thedevice may therefore be used as a polarizer. The reflected beam willalways be essentially polarized at 45° incidence. If the reflectioncoefficient for the TM wave is high, which can be arranged with enoughgrating periods and a high applied field, the extinction ratio of thepolarizer can also be very high in the forward direction. At normalincidence, of course, there is no difference in reflection between thetwo polarizations due to this effect (although there are differences dueto other effects such as the different electro-optic coefficientsdescribed above). A total internal reflection device operating atgrazing incidence is far from Brewster's angle and has little differencein reflection due to this effect.

The wafer material can be any polable solid dielectric material,including ferroelectrics, polymer films, and some amorphous materialssuch as fused silica which can also be poled for producing many usefuldevices according to the invention. The poled material may also be athin film deposited on a substrate of a second material. Many of thepolable thin films, such as fused silica, lithium niobate, potassiumniobate, barium titanate, zinc oxide, II-VI materials, and variouspolymers, have been successfully deposited on a substrate. A widevariety of substrates have been used, including MgO, silicon, galliumarsenide, lithium niobate, and various glasses, including quartz andfused silica. For the domains to be electronically switchable, they mustconsist of electro-optic materials, which are materials having an indexchange induced by an applied electric field.

After the poling step, the liquid electrode material and silica maskingfilm are preferably removed. Referring again to FIG. 2, a firstelectrode 24 and a second electrode 26 confront the dielectric materialin order to provide a means to create the electric field which controlsthe grating. (Confronting a material means placed close to the materialbut not necessarily touching, approximately aligned to the surface ofthe material but not necessarily with a constant gap dimension, andincludes situations with additional material of varying dimensionsplaced on top of the material.) The electrodes 24 and 26, consisting ofan electrically-conductive material, are preferably laid out on opposingsurfaces of the crystal in a spatially delimited manner using standarddeposition techniques. These electrodes are referred to as being onopposing planes even though the surfaces may be curved and/ornon-parallel as pan of a larger geometry. The electrodes may be formedby any material that provides sufficient transport of electrical chargeto achieve an adequate field strength to activate the poled grating in atime consistent with the application. For example, the electrodes couldalternatively consist of metals such as aluminum, gold, titanium,chromium, etc., conductive paint, epoxy, semiconducting material, oroptically transparent materials such as oxides of indium and tin, andliquid conductors such as salt solutions. They may also confront thesurfaces 23 and 25 with a gap filled with air, an optically transparentbuffer layer, and/or other material. Only one electrode is requiredsince a potential voltage difference can be created between thatelectrode and any potential reference such as an exterior ground plane,a second electrode, or multiple electrodes. The electrodes are theelectric field creating means because the application of a voltage to anelectrode establishes an electric field pattern which is determined bythe electrode. A voltage and current supply is of course also needed.The electrodes are placed so that the control electric field is appliedthrough the active volume of the invention, which may consist of apattern poled region or a grating.

In the case of metallic electrodes, it may be best to incorporate acoating deposited below the electrode, to reduce the optical loss whichoccurs when a portion of the guided wave mode extends to the metallicelectrode. The coating should be thin enough to maintain high electricfield at the surface in the case of multiple electrodes mounted on thesame surface, but thick enough to reduce the optical loss. Anothercoating is also useful above the electrodes to reduce the probability ofbreakdown.

A voltage control source 32 (or potential source) provides theelectrical potential to drive the electrodes through connections 30 toactivate the grating. The activated electrodes are polarized relative toeach other according to the polarity of the applied voltage. The voltageof the source produces a large enough electric field through the poledregions to switch a significant amount of light into the switched outputbeam 44. The voltage of the source is variable to provide a means tocontrol the ratio of power in the two output beams. Substantially all ofthe input beam may be reflected with a long grating if the electricfield is sufficiently high, forming an electrically activated mirror.For lower electric fields, the grating forms a partial reflector. Thevoltage control source may be a battery, an electrical transformer, agas powered generator, or any other type of controllable source ofelectrical current and potential. The control means 32 may alsoincorporate a controller which generates a time dependent voltage, andwhich supplies the current to change the voltage on the electrodes 24and 26 at the frequencies required by the application. The control means32 may also have multiple outputs capable of controlling multipledevices, and which might be sequenced temporally according to somepattern. The source 32 may have control inputs for manual or electroniccontrol of its function by computer or by another instrument.

In order to avoid necessary repetition, it should be understood that thevariations described in reference to FIG. 2 apply to the embodimentsdescribed below, and that the variations described in reference to thefigures below also apply to FIG. 2.

Referring now to FIG. 3, a guided-wave embodiment of the presentinvention is shown. Specifically, this embodiment is anelectrically-controlled, frequency-selective waveguide retroreflector.All of the optical beams in this device are confined in two dimensionsby an optical waveguide 64, which traverses one surface of the polabledielectric material that forms the substrate 60 of the device 61.

A waveguide is any structure which permits the propagation of a wavethroughout its length despite diffractive effects, and possiblycurvature of the guide structure. An optical waveguide is defined by anextended region of increased index of refraction relative to thesurrounding medium. The strength of the guiding, or the confinement, ofthe wave depends on the wavelength, the index difference and the guidewidth. Stronger confinement leads generally to narrower modes. Awaveguide may support multiple optical modes or only a single mode,depending on the strength of the confinement. In general, an opticalmode is distinguished by its electromagnetic field geometry in twodimensions, by its polarization state, and by its wavelength. Thepolarization state of a wave guided in a birefringent material or anasymmetric waveguide is typically linear polarized. However, the generalpolarization state may contain a component of nonparallel polarizationas well as elliptical and unpolarized components, particularly if thewave has a large bandwidth. If the index of refraction difference issmall enough (e.g. Δn=0.003) and the dimension of the guide is narrowenough (e.g. W=4 μm), the guide will only confine a single transversemode (the lowest order mode) over a range of wavelengths. If thewaveguide is implemented on the surface of a substrate so that there isan asymmetry in the index of refraction above and below the waveguide,there is a cutoff value in index difference or waveguide width belowwhich no mode is confined. A waveguide may be implemented in a substrate(e.g. by indiffusion), on a substrate (e.g. by etching away thesurrounding regions, or by applying a coating and etching away all but astrip to define the waveguide), inside a substrate (e.g. by contactingor bonding several processed substrate layers together). In all cases,we speak of the waveguide as traversing the substrate. The optical modewhich propagates in the waveguide has a transverse dimension which isrelated to all of the confinement parameters, not just the waveguidewidth.

The substrate is preferably a single crystal of lithium niobate, forminga chip which has two opposing faces 63 and 65 which are separated by thethickness of the wafer. The opposing faces need not be parallel or evenflat. The waveguide is preferably formed by a well-established techniquesuch as annealed proton exchange (APE) on face 63. Alternatively, ionsother than protons may also be indiffused or ion exchanged into thesubstrate material. The APE waveguide increases the crystalextraordinary refractive index, forming a waveguide for light polarizedalong the z-axis. For a z-cut crystal, this corresponds to a TMpolarized mode. Waveguides formed by alternate techniques, such astitanium indiffusion in lithium niobate, may support both the TM and TEpolarizations.

Preferably, the waveguide is designed to support only a single lowestorder transverse mode, eliminating the complexities associated withhigher order modes. The higher order transverse modes have differentpropagation constants than the lowest order mode, and higher scatteringloss, which can be problems in some applications. However, multimodewaveguides might be preferred for some applications, such as for highpower propagation.

One alternative configuration is to excite the grating by applyingpressure rather than by directly applying an electric field. The effectof an applied pressure is indirectly the same: by the piezoelectriceffect, the applied stress produces an electric field, which in turnchanges the index of refraction of the domains. However, no sustainingenergy need be applied to maintain the stress if the structure iscompressed mechanically, for example. This alternative, like the othersmentioned herein, apply also to the other similar realizations of theinvention described below.

Once the waveguide dimensions are determined, a photomask for thewaveguide is generated and the pattern is transferred to a maskingmaterial on the substrate, by one of many well known lithographicprocesses. The mask material may be SiO₂, tantalum or other metals, orother acid resisting materials. To fabricate an APE waveguide, themasked substrate material is immersed in molten benzoic acid to exchangeprotons from the acid for lithium ions in the crystal. The resultingstep index waveguide may then be annealed for several hours at around300° C. to diffuse the protons deeper into the crystal and create alow-loss waveguide with high electrical activity coefficients.

In addition to in-diffusion and ion exchange two-dimensional waveguides,planar and two dimensional ridge or strip-loaded waveguides can beformed. Planar waveguides may be formed by depositing the electricallyactive material on a substrate of lower index. Deposition techniques forwaveguide fabrication are well-known and include liquid phase epitaxy(LPE), molecular beam epitaxy (MBE), flame hydrolysis, spinning, andsputtering. Ridge waveguides can be formed from these planar guides byusing processes such as lift-off, wet etch, or dry etch such as reactiveion etching (RIE). Planar guides can also be used in the presentinvention, particularly in devices using a variable angle of diffractionoff the grating.

The grating 62 in this embodiment is disposed normal to the opticalwaveguide 64 which traverses the substrate. The grating is composed of afirst type 66 and second type 68 of domain, which do not necessarilyextend through the substrate. For example, when the active material ispoled using indiffusion or ion exchange, the inverted domains 66typically extend to a finite depth in the material. The partial domainsmay also be formed when the poling is achieved by destroying theelectrical activity of the material (or reducing the electro-opticactivity) by a technique such as ion bombardment or UV irradiation.

The optical input beam 80 is incident on and is coupled into thewaveguide. Coupling refers to the process of transferring power from oneregion into another across some kind of generalized boundary such asacross an interface, or between two parallel or angled waveguides, orbetween a planar guide and a stripe guide, or between single mode andmultimode waveguides, etc. When the grating is on, a portion of theinput beam is coupled back into a retroreflected output beam 82. Whilethe retroreflection of the grating need not be perfect, i.e. the gratingmay reflect the light to within a few degrees of the reverse direction,the waveguide captures most of this light and forms a perfectlyretroreflected beam. The imperfection of the retroreflection results ina coupling loss of the retroreflected beam into the waveguide 64. Whenthe grating is off (when the controlling electrical field is adjusted tothe "off" position in which the index grating has a minimum value nearzero, typically at zero field), the input beam continues to propagate inthe same direction through the waveguide to form a transmitted outputbeam 84. As in the bulk device, the strength of the grating can bevaried with the voltage source 76 to control the ratio of the power inthe two output beams.

A first electrode 70 and second 72 electrode confront opposing faces ofthe dielectric material 60. The substrate is a dielectric because it iscapable of withstanding an applied electric field without damage, but itneed not be a perfect insulator as long as the current flow does notadversely affect the performance of the device. The electrodes may beformed of any electrically conducting material. There must also be ameans for creating an electric field through the dielectric materialusing the first electrode structure.

The electrodes bridge at least two of the elements of the first type ofpoled structure that forms the grating. This means the electric fieldproduced by the electrodes penetrates into at least the two elements.Thus, these elements can be activated by the field. Two wires 74preferably connect the voltage control source 76 to the two electrodesto provide an electric field in the region formed by the intersection ofthe waveguide 64 and the poled structure 62. The wires may be formedfrom any material and in any geometry with sufficient conductivity atthe operating frequency to allow charging the electrodes as desired forthe application. The wires may be round, flat, coaxial cables, orintegrated lead pattern conductors, and they may be resistors,capacitors, semiconductors, or leaky insulators.

Alternately, the electrodes can be arranged in any manner that allows anelectric field to be applied across the electrically active material.For example, the electrodes may be interspersed in different layers on asubstrate, with the active material between the electrodes. Thisconfiguration enables high electric fields to be produced with lowvoltages, and is particularly useful for amorphous active materials,such as silica and some polymers, which can be deposited over theelectrode material.

The poled structure 62 is preferably deeper than the waveguide so thatthe intersection between the waveguide 64 and the poled structure 62 hasthe transverse dimensions of the mode in the waveguide and thelongitudinal dimensions of the grating.

FIGS. 4, 5 and 6 show alternate electrode configurations in which theelectrodes are disposed on a common face of the dielectric material 189.These configurations are especially useful for embodiments of thepresent invention that use a waveguide 180 to guide an optical beam,since the same-surface electrode configurations permit high electricfields at low voltage. These electrode structures are of particularinterest for low voltage control of the grating 182 because of theproximity of the electrodes to the section of the waveguide whichtraverses the grating. In the electrode configuration 186 depicted inFIG. 4, the first electrode 170 and second electrode 172 confront thedielectric material on the same surface. These electrodes are referredto as being on a common plane even though the surface may be curved aspart of a larger geometry. The first electrode is placed above a portionof the waveguide that contains several grating elements, each of whichconsists of alternate regions of a first type of domain 184 and a secondtype of domain 185. The second electrode is positioned around the firstelectrode. The distance between the electrodes along the waveguide isapproximately constant along the axis of the waveguide for cases where auniform field along the axis of the waveguide is desired. The electrodespacing may also be varied to taper the field strength, as shownschematically in the device 188 of FIG. 6. A voltage source 174connected between the two electrodes disposed as shown in FIG. 4, iscapable of generating electric fields between the electrodes. Theelectric field vectors 176 have their largest component perpendicular tothe surface of the material, in the region of the electrically-activewaveguide. For a z-cut ferroelectric crystal such as lithium niobate,this electric field structure activates the largest electro-opticcoefficient r₃₃, creating a change in index for a TM polarized opticalbeam. For an applied electric field of 10 V/μm and an optical beam witha wavelength of 1.5 μm in lithium niobate, the strength of a first ordergrating is 40 cm⁻¹.

A means 178 for contacting the electrodes to a voltage source isrequired for each of the electrode configurations. To form this means,an electrically conducting material, such as a wire, is electricallycontacted between the electrodes on the device and the terminals of thepotential source. In all electrode configurations, each electrodetypically has a section, or pad, or contact, to which the wire iscontacted. The pads are preferably of large enough size to reduceplacement tolerances on the electrical contact means for easier bonding.The wire can then be contacted to the pads using a technique such aswire bonding by ultrasonic waves, heating, or conductive epoxy.Alternately, a spring-loaded conductor plate can be placed in directcontact with the electrode to make the required electrical connection tothe voltage source. In the figures, the electrodes are typically largeenough and function as the contact pads by themselves.

Another realization 187 of the same-surface electrode structure is shownin FIG. 5, wherein the first electrode 171 and second electrode 173 areplaced on either side of the optical waveguide. When an electricpotential is applied across the two electrodes positioned in thismanner, the electric field vectors 177 have their largest componentparallel to the substrate surface. For a z-cut ferroelectric crystal,the electro-optic coefficient that creates a change in index for a TMpolarized optical wave and the applied electric field is r₁₃. For anapplied electric field of 10 V/μm and an optical beam with a wavelengthof 1.5 μm in lithium niobate, the first order grating coupling constantis 12 cm⁻¹.

Alternately, for TE waveguides the active electro-optic coefficients areswitched for the two configurations. For an electric field vectorperpendicular to the surface of the chip, the appropriate coefficient isr₁₃, while for an electric field vector parallel to the surface of thechip, the electro-optic coefficient used is r₃₃. Similar situationsapply for x- or y-cut crystals, or intermediate cuts.

As a further variation of the configuration of FIG. 5, the electrodesare asymmetrically arranged so that one electrode approximately coversthe waveguide 180 and the other electrode is displaced somewhat to theside. In this configuration, the strong vertical field induced under theedges of the adjacent electrodes is made to pass predominantly throughthe waveguide region under one of the electrodes.

In FIG. 6, the electrodes 175 and 179 have a separation from the centerelectrode 181 which is tapered. When a voltage is applied across theseelectrodes, this configuration produces a tapered field strength, withthe strong field towards the right and the weaker field towards theleft. By "tapered" we mean that any parameter has a generalized spatialvariation from one value to another without specifying whether thevariation is linear or even monotonic; the parameter may be a gap, awidth, a density, an index, a thickness, a duty cycle, etc. The indexchanges induced in the poled domains towards the left of the waveguide180 are therefore weaker than the index changes induced towards theright. This might be useful, for example, to obtain a very narrowbandwidth total reflector where it is needed to extend the length of theinteraction region. In non-normal incidence angle devices, such as shownin FIG. 7 and FIG. 8, the taper might be useful to optimize the couplingof a specific input mode into a specific output mode.

In all electrode configurations, the voltage applied can range from aconstant value to a rapidly varying or pulsed signal, and can be appliedwith either polarity applied between the electrodes. The value of thevoltage is chosen to avoid catastrophic damage to theelectrically-active material and surrounding materials in a givenapplication.

When a constant electric field is applied across materials such aslithium niobate, charge accumulation at the electrodes can cause DCdrift of the electric field strength with time. The charges can bedispersed by occasionally alternating the polarity of the voltagesource, so that the electric field strength returns to its full value.If the time averaged electric field is close to zero, the net chargedrift will also be close to zero. For applications sensitive to suchdrift, care should be taken to minimize the photorefractive sensitivityof the material, such as by in-diffusion of MgO, and operation ispreferably arranged without a DC field.

Surface layers are useful for preventing electric field breakdown andlossy optical contact with the electrodes. Losses are particularlyimportant for waveguide devices, since the beam travels at or near thesurface, while breakdown is most critical when electrodes of oppositepolarity are placed on the same surface. This concern applies to thepoling of the active material as well as to the electro-optic switching.The largest vector component of the electric field between twosame-surface electrodes is parallel to the surface of the material. Boththe breakdown problem and the optical loss problem can be considerablyreduced by depositing a layer of optically transparent material with ahigh dielectric strength between the guiding region and the electrodes.Silicon dioxide is one good example of such a material. Since there isalso a potential for breakdown in the air above and along the surfacesbetween the electrodes, a similar layer of the high-dielectric-strengthmaterial can be deposited on top of the electrodes.

FIG. 7 and FIG. 8 show two embodiments of a electrically-controlledfrequency-selective waveguide coupler. In FIG. 7, a pair oftwo-dimensional waveguides traverse one face of a dielectric material,and intersect at an angle 118 to make a tee, forming a three-portdevice. A grating 100, consisting of a first type 104 and second type102 of domains, is disposed at an angle to the two guides in theintersection region between them (the volume jointly occupied by theoptical modes in the two waveguides). The peak index change in theintersection region is preferably equal to the peak index change in thewaveguides. This is done if the fabrication of the tee structure isaccomplished in one step (be it by indiffusion, ion exchange, etching,etc.). In the alternative approach of laying down two waveguides insubsequent steps, which is most convenient in the crossing waveguidegeometry of FIG. 8, the peak index change in the intersection region istwice the index change in the waveguides, which is not needed. Asalways, the periodicity and angle of the grating is chosen such that thereflection process is phase matched by the momentum of a virtual photonwithin the bandwidth of the grating. For optimal coupling between anin-band input beam in the first waveguide and an output beam 114 in thesecond waveguide 108, the angle of incidence of the input beam is equalto the angle of diffraction off the grating. In this case, the bisectorof the angle between the two guides is normal to the domain boundariesof the grating in the plane of the waveguide.

An input beam 112 is incident on and is coupled into the first waveguide106. A first electrode 120 and second electrode 122 are laid out on thesame face of the dielectric material so that an electric field iscreated in the intersection region between the waveguides, when avoltage source 124 connected to the two electrodes by conductors 126 isturned on. The electric field controls the strength of the grating inthe intersection region via the electro-optic effect, coupling thein-band beam from the first waveguide into the second waveguide to forma reflected output beam 114. With the grating turned off, the input beamcontinues to propagate predominantly down the first waveguide segment toform a transmitted output beam 116 with very little loss. Alternately,counter-propagating beams can be used in the waveguide so that the inputbeam enters though the second waveguide 108, and is switched into theoutput waveguide 106 by interacting with the grating.

In single mode systems, the grating strength is preferably spatiallydistributed in a nonuniform manner so that a lowest order Gaussian modeentering waveguide 106 is coupled into the lowest order Gaussian mode ofwaveguide 108. The grating strength can be modulated by adjusting thegeometry of the electrode, by adjusting the gaps between the electrodes,and by adjusting the duty cycle of the grating. The bandwidth of thegrating may also be enhanced by one of a number of well known techniquessuch as chirping, phase shifting, and the use of multiple periodstructures.

The size of the coupling region is limited, in the geometry of FIGS. 7and 8 by the size of the intersection region between the guides wheretheir modes overlap. To obtain a high net interaction strength for agiven electric field strength, it is desirable to increase the size ofthe waveguides to produce a larger intersection. However, largewaveguides are multimode, which may not be desirable for someapplications. If adiabatic expansions and contractions are used, theadvantages of both a large intersection region and single modewaveguides can be obtained simultaneously. The input waveguide 106begins as a narrow waveguide and is increased in width adiabatically asthe intersection region is approached. The output waveguide 108 has alarge width at the intersection to capture most of the reflected light,and it is tapered down in width adiabatically to a narrow waveguide. Theidea of adiabatic tapering of an input and/or an output waveguide can beapplied to many of the interactions described herein,

Referring to FIG. 8, the two waveguides 136 and 138 intersect at anangle 158 to make an x intersection, forming a four-port device. Thisdevice is a particularly versatile waveguide switch, since two switchingoperations occur simultaneously (beam 142 into beams 146 and 148, andbeam 144 into beams 148 and 146). The grating 130, consisting of a firsttype 134 and second type 132 of domains, is disposed at an angle to thetwo guides in the intersection region between them. The angle of thegrating is preferably chosen such that the bisector of the angle betweenthe two guides is normal to the domain boundaries of the grating, in theplane of the waveguide.

A first input beam 142 is incident on and is coupled into the firstwaveguide 136 and a second input beam 144 is coupled into the secondwaveguide 138. A first electrode 150 and second electrode 152 are laidout on the dielectric material so that an electric field is created inthe intersection region between the waveguides, when a voltage source154 connected between the two electrodes is turned on. The electricfield controls the strength of the index grating in the intersectionregion through the electro-optic effect. When the grating is on, aportion of the in-band component of the first input beam is coupled fromthe first waveguide to the second waveguide to form a first output beam146. At the same time, a portion of the in-band component of the secondinput beam from the second waveguide is coupled into the first waveguideto form the second output beam 148. In addition, the out-of-bandcomponents of the two beams, and any unswitched components of thein-band beams, continue to propagate down their respective waveguides toform additional portions of the appropriate output beams. Thus, for twobeams with multiple optical frequency components, a single frequencycomponent in the two input beams can be switched between the two outputbeams.

The waveguide may only be a segment, in which case it is connected toother optical components located either off the substrate, or integratedonto the same substrate. For example, the waveguide segment could beconnected to pump lasers, optical fibers, crossing waveguides, otherswitchable gratings, mirror devices, and other elements. An array ofcrossing waveguide switches would comprise an optical switching network.

In FIG. 9, a further embodiment of the waveguide coupling switch isshown. The domain walls of the grating are now disposed at a non-normalangle to the surface 157 of the crystal 158, so that the input beam 159in waveguide 160 is reflected out of the plane of the crystal to form areflected output beam 161. As before, an unreflected beam continues topropagate through the waveguide to form a transmitted output beam 162.An optically transparent first electrode 163, which can consist ofindium tin oxide, is disposed on one face of the dielectric material158, over a portion of the grating that crosses the waveguide. A secondelectrode structure 164, which may be optically absorbing, is disposedon the material. As in all cases described in this disclosure, thesecond electrode may be arranged in one of many alternateconfigurations: surrounding the first electrode as in FIG. 7, onopposite sides of the material 158 as on FIG. 2, tapered similar to theconfiguration shown in FIG. 6. The electrodes are connected with twowires 156 to a voltage source 154, which controls the power splittingratio of the in-band beam between the transmitted beam 162 and thereflected beam 161. Alternately, the electrode configuration could be asshown in FIG. 5, in which case both electrodes may be opaque.

Referring again to FIG. 9, the domain walls are preferably formed byelectric field poling of a ferroelectric crystal which is cut at anangle to the z-axis 165. Since the electric field poled domains travelpreferentially down the z axis, poling an angle-cut crystal by thistechnique results in domain boundaries parallel to the z axis, at thesame angle to the surface. The angle 166 of the cut of the crystal ispreferably 45° so that light propagating in the plane of the crystal maybe reflected out of the substrate normal to the surface of the material(any angle may be used). The domains shown in FIG. 9 are planar, but canalso be configured in more general configurations. A planar grating willproduce a flat output phase front from a flat input phase front. If thedevice shown is used as a bulk reflector without the waveguide, acollimated input beam will produce a collimated output beam. The deviceis useful as a bulk reflector for example if a beam is incident fromoutside the device, or if the waveguide is brought to an end within thedevice with some distance between the end of the waveguide and the poledreflector. In some cases, however, it may be desirable to produce acurved output phase front from a collimated beam, as in the case of someapplications requiting focussing, such as reading data from a disk. Bypatterning a set of curved domains on the upper surface of the substrateillustrated in FIG. 9, a set of curved domains may be poled into thebulk of the material since the domain inversion propagatespreferentially along the z axis. A concave (or convex) set of domainsmay therefore be formed which create a cylindrical lens when excited bya field. Wedges and more complicated volume structures oriented at anangle to the surface may be formed by the same process.

In an alternate method, a z-cut crystal can be used as the substrate ifthe poling technique causes the domain boundaries to propagate at anangle to the z-axis. For example, titanium (Ti) indiffusion in a z-cutcrystal of lithium niobate produces triangular domains that would beappropriate for reflecting the beam out of the surface of the crystal.The angle of the domains formed by in-diffusion with respect to thesurface is typically about 30°, so that an input beam incident on thegrating will be reflected out of the surface at an angle of about 60° tothe surface of the crystal. The output beam may then be extracted with aprism, or from the rear surface (which may be polished at an angle)after a total internal reflection from the top surface.

The electrode structure shown excites both an E₃ component, and eitheran E₁ or an E₂ component. A TM polarized input wave 159 experiences anindex change which is a combination of the extraordinary and theordinary index changes.

In FIG. 10 there is shown an embodiment of a switchable waveguidedirectional coupler. A first waveguide 204 is substantially parallel toa second waveguide 206, over a certain length. While the beams propagateadjacent each other and in a similar direction, their central axes aredisplaced. The central axes are never brought coaxial so that thewaveguides do not intersect. However, the waveguide segments are inclose proximity in a location defined by the length of the coupler, sothat the transverse profiles of the optical modes of the two waveguidesoverlap to a large or small extent. The propagation of the two modes isthen at least evanescently coupled (which means the exponential tailsoverlap). The evanescent portion of the mode field is the exponentiallydecaying portion outside the high index region of the waveguide. Thepropagation constant associated with a mode of each of the twowaveguides is determined by k=2πn_(eff) /λ in the direction ofpropagation. The effective index n_(eff) is the ratio of the speed oflight in a vacuum to the group velocity of propagation, which variesaccording to the mode in the waveguide. The value of n_(eff) isdetermined by the overlap of the mode profile with the guided wavestructure.

Preferably, the width of the two waveguides, and thus the propagationconstants of the modes in the two waveguides, are different, so thatcoupling between the modes is not phasematched when the grating is off.(The index of refraction profiles of the two waveguides may also beadjusted to create different propagation constants.) With the gratingoff, any input beam 210 in the first waveguide will continue topropagate in that waveguide to form a transmitted output beam 214exiting the first waveguide 204. When the grating is on, the gratingmakes up the difference in the propagation constants of the twowaveguides so that coupling between the two modes is phasematched, andan in-band output beam 212 exits the second waveguide 206. To optimizethe coupling, the grating period Λ is chosen so that the magnitude ofthe difference of the propagation constants in the two waveguides isequal to the grating constant (within an error tolerance). Thepropagation constants of the two waveguides may alternately be chosen tobe equal, so that coupling between the two waveguides occurs when thegrating is off. In this case, turning the grating on reduces thecoupling between the two guides.

The strength of the grating determines a coupling constant, whichdefines the level of coupling between the two waveguides. Along thelength of the interaction region of the two waveguides, the powertransfers sinusoidally back and forth between the guides, so thatcoupling initially occurs from the first waveguide to the second, andthen back to the first waveguide. The distance between two locationswhere the power is maximized in a given waveguide mode is known as thebeat length of the coupled waveguides. The beat length depends on thestrength of the grating.

A first electrode 220 and second electrode 222 are positioned on thematerial surface to create an electric field across the grating region202 when a voltage is applied between the two electrodes. A voltagesource 226 is connected to the two electrodes with an electricallyconductive material 224. The strength of the grating, and thus the beatlength between the two waveguides, is controlled by the voltage appliedacross the grating.

The propagation constants of the two guides are strongly dependent onwavelength. Since the momentum of the virtual photon is essentially ordominantly fixed (i.e. determined by parameters which are not varied inan application), power is transferred to the second waveguide only inthe vicinity of a single frequency with a frequency bandwidth dependingon the length of the coupling region. Depending on the grating strength,an adjustable portion of the in-band input beam exits the secondwaveguide as the coupled output beam 212, while the out-of-band portionof the input beam exits the first waveguide as the transmitted outputbeam 214 along with the remainder of the in-band beam.

The coupling between the two modes can be controlled electro-opticallyby several means, including changing the strength of the couplingbetween the modes, increasing the overlap of the modes, or changing theeffective index of one of the waveguides. Electro-optically controlledcoupling, described above, is the preferable method. In order to coupleefficiently between the modes in the two waveguides, the input beam isforward-scattered, which requires the smallest grating period.

The coupling grating can alternatively be implemented as a combinationof permanent and switched gratings as described above in conjunctionwith FIG. 2. Here we give a detailed example of how this can be done.After forming the desired periodic domains, the substrate can bechemically etched to form a relief grating with exactly the same periodas the poled structure. For the preferred material of lithium niobate,the etch can be accomplished without any further masking steps, sincethe different types of domains etch at different rates. For example,hydrofluoric acid (HF) causes the -z domains of lithium niobate to etchsignificantly (>100×) faster than the +z domains. Thus by immersing thez-cut crystals in a 50% HF solution, the regions consisting of the firsttype of domain are etched while the regions consisting of the secondtype of domain essentially remain unetched. This procedure produces apermanent coupling grating which can be used on its own to producecoupling between the two waveguides. After the electrodes are applied,the poled grating can be excited to produce an additive index ofrefraction grating which is superimposed on that of the etchedsubstrate. The etch depth may be controlled so that the effective indexchange induced by the permanent etched grating can be partially orwholly compensated by the electro-optically induced grating when theelectrodes are excited at one polarity, while the index grating isdoubled at the other excitation polarity. A push-pull grating is therebyproduced whereby the grating can be switched between an inactive stateand a strongly active state.

An etched grating is also useful when the etched region is filled withan electro-optical material, such as a polymer or an opticallytransparent liquid crystal, with a high electro-optic coefficient and anindex close to that of the substrate. Preferably, the filled etchedregion extends down into the optical beam. When a voltage is appliedacross the filled etched region, the index of the filler material isalso varied around that of the rest of the waveguide.

Alternately, the overlap of the modes in the two waveguides can beelectro-optically modified. For example, the region between the twowaveguides could have its refractive index raised. This reduces theconfinement of the waveguides, and spreads the spatial extent of theindividual modes towards each other, increasing the overlap. Toimplement this approach, the region between the two waveguides may bereverse poled with respect to the polarity of the substrate traversed bythe waveguides. If the electrode extends across both the waveguides andthe intermediate region, an applied voltage will increase the index ofthe area between the waveguides while decreasing the index within thetwo waveguides. The resulting reduction in mode confinement thusincreases the overlap and the coupling between the two modes. Care mustbe taken not to induce undesirable reflections or mode coupling loss inthe waveguides, which might occur at the edge of the poled region. Theselosses can be minimized, for example, by tapering the geometry of thepoled regions or of the electrodes so that any mode change occursadiabatically along the waveguide, minimizing reflections. An adiabaticchange means a very slow change compared to an equilibrium maintainingprocess which occurs at a definite rate. In this case, it means thechange is slow compared to the rate of energy redistribution whichoccurs due to diffraction within the waveguide and which maintains thelight in the mode characteristic to the waveguide.

A third means to change the coupling between the two waveguides is tochange the effective index of one of the waveguides relative to theother. Thus, the propagation constant of the guide is changed, which inturn alters the phasematching condition. This effect may be maximized bypoling one of the waveguides so that its electro-optic coefficient hasthe opposite sign from that of the other waveguide. In this case, thecoupling grating may be a permanent or a switched grating. A firstelectrode covers both waveguides and the region between them, while asecond electrode may be disposed on both sides of the first electrode.An electric field applied between the two electrodes causes thepropagation constant of one waveguide to increase, and that of the otherwaveguide to decrease, thus maximizing the difference in propagationconstants. The grating coupling process is maximally efficient only at aparticular difference in propagation constants. By tuning the appliedvoltage, the phasematching may be adjusted as desired. This effect canbe used to create a wavelength tunable filter.

The parallel waveguides shown in FIG. 10 may be nonparallel, and thewaveguides may not even be straight. If it is desired, for instance, tospatially modify the interaction strength between the waveguides, thisend can be accomplished by spatially adjusting the separation betweenthe guides. These modifications may also, of course, be applied to thesubsequent embodiments of parallel waveguide couplers described herein.

Referring to FIGS. 12 and 13 there are shown alternate embodiments ofthe crossing waveguide coupler for controlling the profile of thereflected beam. In each embodiment, the area covered by the grating doesnot extend entirely across the intersection region of the twowaveguides. The motivation for these grating structures is bestunderstood with reference to FIG. 11. Depending on how it is configured,the power coupling structure 282 may distort the spatial profile of themode 284 it couples into the output waveguide. A power coupler which isuniform in space and which uniformly covers the entire intersectionregion 280 between two waveguides disposed at a large angle to eachother such as 90° will produce an output beam profile such asassymmetric profile 286. The power in the input beam decreases as itpasses through the power coupling structure or grating. In the case of aright angle intersection, the near field profile of the reflected beammatches the monotonically decreasing power in the input beam. Thedisadvantage with the nonsymmetric profile 286 lies in single modestructures where only a fraction of the coupled power will remain in thewaveguide. Much of the power will be lost from the guide.

For single mode devices, a structure is needed which couples power intothe Gaussian-like spatial configuration 288 of the lowest order mode ofthe output waveguide. To accomplish this goal, the region 282 must beextended out into the evanescent tails of the guided modes, and the netinteraction must be modulated, either geometrically or by spatiallyadjusting the local strength of the power coupling grating. FIGS. 12 and13 show ways to accomplish this end with geometrical arrangements ofgratings. It is also possible to accomplish this end by spatiallymodulating the "duty cycle" of the grating within the power couplingregion 282, by changing the order of the grating in selected regions,and in the case of electrically controlled coupling, by tapering thestrength of the applied electric fields (by adjusting electrode spacingas illustrated in FIG. 6, or by adjusting the electrode duty cycle inthe case of grating electrode structures). The duty cycle of a gratingmeans the fraction of each period which is occupied by a given domaintype; the duty cycle may vary with position.

In FIG. 12, a device 300 with a modified grating structure is shown, inwhich the grating area 310 covers part, but not all of the rectangularintersection region of the two normal guides 316 and 318. With thegrating unactivated, the input beam 302 passes through guide 316undeflected to exit as output beam 308. The dimensions of theintersection region match the widths 304 and 305 of the two waveguides.The presence of a small region of power coupling structure at any pointin the intersection region will result in local coupling between a giventransverse segment of the beam profile in an input waveguide into agiven transverse segment of the beam profile in an output waveguide. Thereflected beam profile is constructed from the propagated sum of thesephased-coupled contributions. The grating region 310 depicted istriangular in shape, with the points of the triangle 311, 312, and 313.The shape of the grating region can be modified from the triangular, andthe local grating strength can be modulated. The exact shape of thegrating region which optimizes single mode coupling characteristicbetween the waveguides can be calculated with an established waveguidepropagation technique, such as the beam propagation method.

A further embodiment of a single-mode coupling grating device 340 isshown in FIG. 13. The grating region 350 is a double convex shape, withone point at corner 351 common with waveguides 346 and 348 and beams 330and 342, and the other point on opposite corner 352, common with bothwaveguides and beams 342 and 332. This structure has the advantage ofreflecting most of the power in the middle of the beam, where theoptical intensity is the highest, and thus better couples the powerbetween the lowest order modes in the two waveguides 346 and 348. Theoptimal shape of the grating region again depends on the couplingconstant of the grating.

Referring to FIGS. 12 and 13, a first electrode 320 is disposed on thesame surface of the substrate as the waveguide, over the grating region,and a second electrode 322 is disposed on the same surface around thefirst electrode. The distance between the two electrodes may be constantas illustrated in FIG. 13, or it may be tapered as illustrated in onedimension in FIG. 12. A voltage control source 324 is connected with twowires 326 to the two electrodes. An electric field can thus be appliedthrough the grating region to activate one of the electro-opticcoefficients and change the coupling between the input beam and theoutput beam.

For purposes of illustration, FIG. 12 also shows a tapered inputwaveguide segment 287 and a tapered output segment 289. An input beam285 expands adiabatically through the tapered segment 287 to increasethe intersection area and thereby increase the total reflection fromgrating 310. The grating is capable of reflecting the now-expanded beam285 toward the output beam 308. If desired, the output waveguide mayalso contain a tapered segment 289 to reduce the witdth of the outputbeam. (Alternatively, the output beam may be kept wide if desired forlater beam switching interactions.)

The grating may extend beyond the intersection region of the twowaveguides. A grating extended along the input waveguide enablesresidual transmitted light after the intersection region to be removedfrom the waveguide, typically into radiation modes. The extended gratingminimizes crosstalk between optical channels in switching arrays, inwhich an individual waveguide may have more than one signal channelpropagating along its length.

Specifically contemplated by the invention is a means for tuning thegrating. Several embodiments in which tuning is achieved are shown inFIGS. 14-17. Referring to FIG. 14, there is a bulk optical device 400 inwhich the strength and center wavelength of a normal incidencereflection grating are controlled by a single voltage source 426. Thisdevice consists of a patterned poled grating region 410, which iselectro-optically activated by two electrodes 420 and 422 on opposingsurfaces of the material and connected to 426 by conductors 424. Thestrength and the center frequency of the grating are tunedsimultaneously by applying a single voltage between the two electrodesof the device. The average refractive index of the grating changes withthe applied electric field, causing a change in the center wavelength ofthe grating that is proportional to the electric field. The averageindex is calculated over a single period of the grating in a periodicgrating, by summing the weighted index changes in the various types ofdomains. Tho weighting factor is the physical length 416 and 418 of eachdomain type, along the optical path of the input beam 404. The conditionfor frequency tuning is that the weighted sum must not equal zero sothat the average index changes as a result of the electric field.

The product of the index of refraction and the physical distancetraversed by an optical beam is known as the optical distance. (Theindex of refraction is replaced by the effective index of refraction forwaveguide devices.) A 50% duty cycle is obtained in a grating with twotypes of domain if the average optical distance across the two types ofdomains is substantially equal (approximately equal within the errorrange determined by the needs of the application). The average is takenover many subsequent domains to allow for the possiblity of a chirped,nonperiodic, or other more general type of grating. In general thedomains may have different indices of refraction as well as differentelectro-optic coefficients. The general condition for tuning isexpressed in terms of the physical distance travelled in the differenttypes of domains. For each domain, the total optical phase advance isgiven by the optical distance travelled (times 2π/λ). However, thechange in the phase advance is given by the product of the appliedelectric field, the appropriate electro-optic coefficient, and thephysical distance (times 2π/λ). The average change in index ofrefraction experienced by the wave is equal to sum of the changes inphase advance in all domains traversed by the optical wave within asection of the material of length l (times λ/2πl). This change inaverage index determines the change in the peak interaction wavelengthaccording to δλ/λ=δn/n. The grating strength is changed simultaneouslywith the wavelength in this structure, but such simultaneous change maybe undesirable. The structure may be designed so that the operatingpoint about which tuning is accomplished maintains a sufficiently highgrating strength for the application across the entire wavelength tuningrange. Or, a separate tuning structure may be used as is described belowin reference to FIGS. 16 and 17.

The change in the average refractive index can be achieved by manydifferent means. One alternative is that of randomlynon-electro-optically active domains 414 alternating withelectro-optically active domains 412. The electro-optically activeregions are poled domains, while the non-electro-optically activedomains may be randomly poled or unpoled or radiation-disabled. Thus,the electric field causes an average increase in the index Δn_(avg)across the grating. In the poled-random configuration of FIG. 14,Δn_(avg) is equal to the product of the index change in the activedomains 412 times the duty cycle. The duty cycle is equal to the length418 divided by the sum of the lengths 418 and 416. The tunability thatcan be achieved using this technique is λΔn_(avg) /n in a poled-randomstructure, where λ is the optical wavelength, and n is the original(effective) index of the material. Assuming a wavelength of 1.55 μm anda 10 V/μm electric field in lithium niobate, the tuning range for a 50%duty cycle structure is 1.1 nm.

When the input beam 404 is within the bandwidth of the grating, thegrating couples the beam into a retroreflecting output beam 402;otherwise the input beam forms a transmitted output beam 406. Contrastthis behavior with that of a 50% duty cycle grating where the two domaintypes have the same electro-optic coefficients but opposite polarity, asin the case of domain inversion. In this latter case, there is no changein the average index of refraction since the change in index of thefirst domain type cancels with the change in index of the other domaintype. A 50% duty cycle domain reversal grating does not tune its centerfrequency.

An alternate means to achieve an average effective index change indomain reversed gratings is to use a non-50% duty cycle for the poleddomain area with unequal lengths 416≠418. The tunability that can beobtained using this technique is (2D-1)Δnλ/n, where D is the duty cycleof the largest domain type (D>0.5). For example, with a 75% duty cycle,a wavelength λ of 1.5 μm, and a 10 V/μm electric field in lithiumniobate, the tuning range is 0.54 nm. The domain reversed grating isalso stronger than a grating in which the second domain type is notelectro-optically active.

In FIG. 15, a waveguide device 440 using the same average index effectis shown. In this case, the average effective index of the waveguide 442in the grating region 450 changes with the applied electric field,causing a change in the center wavelength of the grating. A voltagecontrol source 466 is used to apply an electric field between a firstelectrode 460 and second electrode 462, which are preferably placed onthe same surface of the material. The average effective index can beachieved by a variety of geometries, including non-electro-opticallyactive domains or a domain reversal grating with a non-50% duty cycle.When the input beam 445 is within the bandwidth of the grating, thegrating couples the beam into a retroreflecting output beam 444;otherwise the input beam forms a transmitted output beam 446.

A means to enhance the tunability of a grating in a waveguide device 480is to overlay a second electro-optic material 482 on the waveguide toform a cladding, as shown in FIG. 16. The cladding should be transparentto the wave propagating in the waveguide and it should be electricfield-sensitive to enable adjustable modification of its index ofrefraction. The average effective index is determined partly by theindex of refraction of the cladding. The second material may have ahigher electro-optic coefficient than the substrate. Liquid crystals andpolymers are good examples of materials which can be used as cladding.The index of the cladding is preferably close to that of the guidingregion so that a large portion of the guided beam propagates in thecladding.

For this embodiment, a first electrode 502 is surrounded by a secondelectrode 504 on the substrate, for applying an electric field acrossthe poled grating 490. Preferably, the electrodes are placed below thecladding, directly on the substrate. If the first electrode 502 ispositioned directly above the waveguide 484 as shown in FIG. 16, it mustbe made of an optically transparent material. The electrodes may also bedisposed to either side of the waveguide 484, in which case they neednot be transparent. A third electrode 506 is positioned on top of thecladding, above the waveguide and the first electrode. For thisembodiment, the center wavelength and strength of the grating areseparately controllable. The grating strength is controlled by a firstvoltage source 510, connected by two wires 513,514 to the first andsecond electrodes, while the center wavelength of the grating iscontrolled by a second voltage source 512, connected between the firstand third electrodes with two wires 514 and 515. In an alternateelectrode configuration, only two electrodes are used, both of which arepreferably positioned on top of the cladding material so that theirinduced field penetrates both the cladding material above the grating,and the grating structure itself. A single voltage source then controlsboth the center wavelength and the grating strength, but notindependently.

The amount of tunability that can be achieved with an electro-opticallyactive cladding depends on what portion of the guided beam propagates inthe cladding. If the two indices are relatively close so that 10% of thebeam propagates in the cladding, then the average change in theeffective index of the guided mode is equal to 10% of the change inindex of the cladding. For a cladding index change of 0.1, thetunability is on the order of 7 nm.

FIG. 17 shows an embodiment of a discretely tunable grating device 520,which consists of several individually controllable gratings 530, 532,534. The gratings in series, with all gratings in the path of the inputbeam 522, and forward 523 and reflected 524 beams. Each individualgrating in the structure may also be continuously tunable over a smallrange. Each grating in FIG. 17 has a first electrode 542 and a secondelectrode 544, which are connected to a voltage controlling network 552with wires. The gratings can be switched on one at a time, so that onlyone wavelength in a small passband will be reflected at a time, ormultiple gratings can be switched on simultaneously to create aprogrammable optical filter, with a center wavelength and bandwidthwhich are separately controlled. The gratings themselves may beimplemented with the variations described above, including thepossibility of multiple periods in each grating.

The structure can be realized either in the bulk or as a waveguidedevice. In the latter case, an optical waveguide 528 is fabricated onthe substrate so that the waveguide intersects the poled gratings. Thepoled domains 536 may extend only through the waveguide and do notnecessarily extend all the way through the material. Both electrodes arepreferably (for higher field strength) deposited on the same face of thesubstrate as the waveguide. The second electrodes of all the gratingsmay be connected as shown to minimize the number of electricalconnections.

Alternately, the individually-addressable grating structure can be abulk device, in which case the waveguide 528 is omitted, and the poledregions 530, 532 and 534 are optimally fabricated with sufficient depthto overlap with the propagating optical mode. The two electrodes forcontrolling each grating are then optimally positioned on opposing facesof the material to optimize the field penetration, as shown for examplein FIG. 2 for a single grating. Cross excitation between adjacentgratings caused by fringing of the electric fields between theelectrodes can be minimized by separating the grating-electrode groupsby an amount comparable to the substrate thickness, or by addinginterspersed fixed-potential electrodes.

An alternate means for tuning the grating is to vary the temperature ofthe active material. The tuning occurs because of two effects: thermalexpansion and the thermo-optic effect. For different materials, eitherone of these two effects may dominate thermally induced tuning. Inlithium niobate, the larger effect is thermal expansion, for which thelargest (a-axis) expansion coefficient ΔL/L is +14×10⁻⁶ °C⁻¹, while thethermo-optic coefficient for the ordinary axis Δn_(o) /n is +5.6×10⁻⁶°C⁻¹. For a temperature range of 100° C., the combination of these twoeffects gives a total wavelength tuning range of 2.6 nm.

For many purposes, it is desirable to create poled gratings with ageneralized frequency content. Multiple interaction peaks may be desiredfor example, or simply a broadened bandwidth of interaction. Toaccomplish this end, some way is needed to determine the pattern ofpoled region boundaries which corresponds to a given mathematicalfunction containing the desired frequencies. FIG. 18 illustrates theresults of the process in the case of a single frequency containingarbitrary phase shifts. Referring now to FIG. 18, optical phase shifts564 and 565 can be incorporated at one or more positions along asinusoidal function 560 to modify its wavelength structure. The meanlevel of the function is given by the straight line 561. Also shown isthe corresponding squared wave function 562 with identical phase shifts,as can be achieved by a typical poling process. To achieve thetranslation of the continuous function into the square wave function,the regions 570 where the curve 560 exceed the average 561 sine wavecorresponds to one type of domain, while the regions 572 where the curve560 falls below 561 corresponds to a second type of domain. The Fouriertransform of the square wave curve 562 will have the same frequencycomponents as the transform of the sinusoidal function 560 in the lowfrequency range below the harmonics of the sine wave frequency. Thisapproach works for any type of generalized frequency distribution aslong as the bandwidth does not exceed a small fraction of the carrierfrequency.

A phase shifted grating may be implemented in any of the devicesdescribed herein such as in FIG. 2 for example, where the location ofthe domain walls 34 in the grating 22 can be determined by the pattern562 of FIG. 18 rather than a periodic function. The phase shiftedpattern can be controlled with a poling mask incorporating the desiredpattern.

Arbitrary multiple period gratings can be specified using a similartechnique. Each period present in the grating is represented in aFourier series (or integral) by a corresponding sign wave of the desiredamplitude. All waves are added together to form a resultant wave. Thepositive portion of the resultant wave corresponds to one type ofdomain, while the negative portion corresponds to the second type ofdomain. The number of superimposed gratings can in principle be scaledup to any number, limited in practice by the minimum attainable featuresize.

FIG. 19 shows an alternate way of fabricating a superimposedmultiple-period grating device 580. A two grating waveguide structure isdepicted, with a switchable single period poled grating 582, and apermanent relief grating 584 interacting with a single beam in awaveguide. A coating 588 is shown deposited on top of the relief gratingto reduce the loss which occurs when the evanescent tail of the guidedwave mode overlaps with the metallic electrode. This coating is animportant design optimization element for all of the elements describedherein, and should be applied between each electrode structure andadjacent optical waveguides. A coating is also useful above theelectrodes in all of the elements described herein to reduce theprobability of breakdown.

The electrically controllable gratings in the superperiod structure areswitched by a single pair of electrodes 602 and 604, connected by wires606 to a voltage control source 608. The first electrode 602 ispreferably centered over the waveguide, while the second electrode 604runs parallel to the first, on either side of the waveguide. The devicedepicted is a waveguide device, with a waveguide 586 confining the inputbeam 590, as well as the transmitted output beam 592 and the reflectedoutput beam 594.

The multiple period grating structure can be configured in many ways.For example multiple independent peaks in the frequency spectrum can beuseful as a multiple frequency feedback mirror. Two operations (e.g.,phasematching and reflection) can be achieved in a single grating whichincorporates the proper two periods for enabling the processes. As afinal example, the grating can be fabricated with the phase andamplitude of its components adjusted for equal effect on the twopolarization modes, making a polarization insensitive component.

Another useful modification of a periodic structure is a chirped period.Along the length of the grating structure, the period can be graduallyincreased or decreased, so that the center wavelength varies from oneend of the grating to the other. Thus, the wavelength bandwidth of thegrating is broadened over that of a constant period grating. Thechirping across the grating is not necessarily linear: many differentwavelength reflection profiles in frequency space (e.g., square wave,Lorentzian) can be achieved, depending on the variation in the chirprate. As described above, the duty cycle and/or the strength of theexciting electric field can also be spatially adjusted to modify thestrength of different portions of the chirped grating. The duty cycle ofthe grating can be controlled by the mask as desired. The electric fieldstrength can be controlled by adjusting the separation of the electrodesas shown for instance in FIG. 6.

A wide spectrum tunable device can be realized in a structure containingtwo separate gratings which have a multiple peak structure, as shown inFIGS. 21 and 22. FIG. 20 demonstrates the basic principle of thesedevices and depicts the multipeak comb transmission (or reflection)profiles 620 and 622 as a function of the optical frequency for two suchgratings. The first grating profile 620 has transmission peaks separatedby a first period 626, while the second grating profile 622 has peaksseparated by a second period 624 that is slightly different from thefirst. The key idea is for the device to operate only at a frequencydetermined by the overlap of peaks from both curves (frequency ν₁).Tuning is achieved by tuning the comb of transmission peaks of thegratings with respect to each other. Different transmission peaks in thetwo combs will overlap each other in various ranges of the relativefrequency shift, so that the net transmission of the combined gratingsjumps discretely over a much wider wavelength range than can be achievedwith only thermal or electro-optic tuning. In the example of FIG. 20where the peak separations differ by 10%, if the frequency of the firstgrating is increased by 10% of the frequency separation 626, the nexthigher frequency peaks will superimpose, resulting in an effectivefrequency shift ten times larger than the tuning amount.

In FIG. 21, a guided wave embodiment of the device is shown, in whichtwo gratings 650 and 652 are placed over a single waveguide 642. Aninput beam 644 is partially reflected into beam 643 and transmitted asbeam 645. A first electrode 666 and second electrode 668 are positionedaround the first grating 650 so that a first voltage source 662connected to the electrodes activates that grating. A third electrode664 is positioned, along with the second electrode, around the secondgrating 652. The second grating is controlled by a second voltage source660 connected to the second and third electrodes. In the preferredembodiment, each grating is a multiple peak structure as described inFIG. 20, and the device forms a frequency-hop-tuned reflector. Accordingto the curves of FIG. 20, the gratings are configured as broadbandreflectors, reflecting essentially all the incident radiationfrequencies except a comb of equally spaced frequencies where thetransmission is high. The cascaded gratings will therefore reflect allfrequencies in the frequency range illustrated in FIG. 20, except wherethe two transmission peaks overlap at ν₁. Provided that the reflectionsof the two gratings are arranged to add in phase in the reflected beam643, the transmitted spectrum will be essentially equal to the productof the two transmission curves 620 and 622. When the center frequency ofone of the gratings is tuned, the single transmission peak at ν₁ willhop to the next adjacent peak, and then the next, and so on. Such astructure is particularly useful as an electrically tuned receiver in,for example, a wavelength-division-multiplexed (WDM) communicationsystem. The receiver can be configured to detect only incoming light ina specific band, while being insensitive to light at other frequencies.

As seen above, a grating structure can be shifted by about 0.5 nm,assuming a 10 V/μm field in a domain inverted grating with duty cycle of75%. This continuous tuning range can be used to produce discontinuoustuning in the structure 640 across perhaps 100 bands in a 50 nm range,if the width of the individual frequency peaks 628 are narrower thanabout 1/100th of the frequency separation.

Note that if the frequency of the input light is known to lie onlywithin the transmission bands of curve 620 in FIG. 20, for example, thedevice can be realized with only a single grating structure with thetransmission spectrum of curve 622, using essentially the Moire effect.By tuning the center frequency of the spectrum 622, any one of thedesired bands can be selected while reflecting the rest. The teestructure of FIG. 7 is then particularly interesting in this context:the input beam 112 containing multiple frequency components is thensplit by the grating structure 100 (configured for tuning as describedherein) into a single transmitted beam 116 which can be detected orotherwise processed, and a reflected beam 114 which contains all theother frequency components. The power contained in beam 114 is not lost,but can be routed to other nodes in a communications network, forexample.

Other variations can be formed of this basic structure, wherein, forexample, the spectra of FIG. 20 are the reflection curves of theindividual gratings instead of the transmission curves. In this case,the structure acts as an etalon when the frequencies of the reflectionpeaks align with each other, with reflectivity according to the relativephase of the reflected waves. Otherwise, the net reflection of thecompound structure is essentially the sum of the reflection curves ofthe two individual structures.

It may be important to optimize the relative phase of the tworeflections by adjusting the optical path length 653 between the twogratings. The relative phase can be controlled by using an electro-opticstructure (as shown for example in FIG. 22) between the two gratingentrances 654 and 655 to adjust the optical path length 653. For alithium niobate crystal and an input wavelength of 1.5 μm, an activateddistance between the gratings of at least 250 μm is required to adjustthe relative phase between the two beam of up to ±π, (using a z-axisapplied field of 10 V/μm). The strength of one of the gratings (but notits frequency) may optionally be controlled via a field applied at itselectrode if the grating is not designed for tuning (its average indexof refraction is configured to be independent of the applied voltage).If both gratings are tuned together, narrow range continuous tuningresults. As an alternative or supplement to electronic excitation, thephase of the two reflections and the peak wavelengths of the gratingscan all be varied together through thermal or mechanical control of thechip.

FIG. 22 shows schematically two grating reflectors 633 and 634 separatedby a phase shifter section 635 and forming an integrated etalon 640having a characteristic free spectral range (FSR). (The structure 630 isessentially the same as that of FIG. 21, with the addition of the phaseshifter section, which consists of electrodes capable of actuating aregion of electro-optic material traversed by the waveguide 636.) Forsimplicity, we consider the case of uniform single-period gratings, butthe individual gratings may generally be more complex structures. Thegratings may be fixed or electronically actuatable. The reflections offthe two gratings can be made to add in phase for a beam at a referencefrequency by adjusting the voltage applied to the phase shifter section635. A beam at a second frequency will also add in phase if thefrequencies of the two beams are separated by a multiple of the FSR.Since the FSR is inversely proportional to the optical path lengthbetween the two gratings, choice of the path length determines thedensity of the reflection peak structure of the etalon device. As anexample, two short high reflecting gratings separated by 220 μm inlithium niobate can have grating reflection peaks separated by amultiple of 1 nm. The multiple peak structures 620 or 622 described inFIG. 20 can each be implemented as an integrated etalon.

A dual grating wye junction embodiment is shown in FIG. 23, in which thetwo gratings 690 and 692 extend across two separate waveguides 682 and684. In general a wye junction has an input and multiple outputwaveguides which may lie in a plane or in a volume. The two waveguidesare connected to the first waveguide 686 with a wye junction 688. Thepower in the optical input beam 691 is split between the secondwaveguide 682 and third waveguide 684 so that approximately 50% of theinput beam 691 is incident on each of the gratings. The two gratings mayhave a simple reflection structure, or they may have a series of highreflection peaks. The gratings may be permanent, or they may beelectronically adjustable, in which case electrodes 694 and 696 areprovided for exciting the gratings. A common electrode 698 is thenprovided across the wafer (or alternately on the same surface as thewaveguides, adjacent to the other electrodes similarly to FIG. 21).

The relative optical path length of the two branches of the waveguidecan be adjusted by the electrode 689 which is disposed on one waveguideover a region of electro-optic activity. By adjusting the voltage on thephase adjusting electrode 689, the two reverse-propagating reflectedbeams may be adjusted to have the same phase when they meet at the wyejunction. The reflected modes superpose and form a wave front profilewhich may have a phase discontinuity in the center, depending on therelative phase of the two waves. As the combined wave propagated, thespatial concentration of the optical mode in the region of the guide isstrongly affected by the phase shift. If they have the same phase, theprofile forms a symmetric mode which couples efficiently into the lowestorder mode of the input waveguide to form the retroreflected output beam693. Two reflected beams which add out-of-phase at the wye junction willhave very low coupling into any symmetric mode (such as the lowest ordermode) of waveguide 686. If the waveguide 686 is single mode, thisreflected energy will be rejected from the waveguide. Thus, by adjustingthe optical path length of one of the arms of the wye with the electrode689, the reflection can be rapidly adjusted from almost 100% to a valuevery close to zero. Furthermore, if the gratings are implemented aselectronically tunable reflectors in one of the tunable configurationsdescribed herein, the modulated reflection property can be shifted intodifferent regions of the spectrum.

Referring to FIG. 24, there is shown a switchable waveguide modeconverter 720 using a poled grating 722. The waveguide 730 preferablysupports both an input mode and an output mode, which may be twotransverse modes or two modes of polarization (e.g. TE and TM). The twomodes in the waveguide typically have different propagation constants,which are determined by the effective indices of the modes. The grating722 is excited electrically by electrodes 740 and 742, coupled to thesource of electrical potential 744 by the connections 746. The gratingperiod Λ (724) is chosen so that the magnitude of the difference of thepropagation constants in the two waveguides is equal to the gratingconstant 2πn/Λ. When the grating is on, the grating makes up thedifference in the propagation constants of the two waveguides so thatcoupling between the two modes is phasematched. The grating strength andthe device interaction length in the grating should be set to optimizethe flow of power from the input mode into the output mode. The net rateof power conversion from one mode into the other is determined by thestrength of the electro-optic coefficient (r₅₁ in lithium niobate) andby the strength of the electric field.

For two transverse modes, the coupling depends on the spatial overlap ofthe two modes in the presence of the grating structure, and on thestrength of the grating. The two modes may be orthogonal by symmetry, sothat even if the modes are phasematched, there will be no conversion ina symmetric structure. In this case, the phasematching structure itselfcan be made asymmetric to eliminate the problem. In the preferredembodiment of FIG. 24, the asymmetry can be introduced via the electricfields which excite the poled structure. The vertical component of theelectric field reverses sign midway between the two electrodes 740 and742. It is best to center the electrodes on the waveguide to optimizemode conversion between transverse modes of different symmetry. Thereverse is true when coupling transverse modes of the same symmetry: nowthe phasematching structure should be made symmetric to optimize theconversion. Several alternative approaches can also be used. A threeelectrode structure has a symmetric vertical component of the electricfield and an asymmetric horizontal field. The horizontal field can beused in conjunction with one of the horizontally-coupled electro-opticcoefficients to couple modes of different symmetry. Or, the poledstructure may have a phase reversal plane that essentially bisects thewaveguide, in which case a symmetric component of the electric field canbe used to couple modes of different symmetry (vertical field in thecase of three electrodes, horizontal field in the case of two).

Since the propagation constants of the two modes are strongly dependenton wavelength, the beat length of their interaction also depends on thewavelength. Thus, for a given length of the coupling region between thetwo modes, the power coupled into the second mode isfrequency-sensitive. The coupling has a frequency bandwidth associatedwith it. For a given grating strength, a portion of the in-band inputbeam is coupled into the output mode which exits as the coupled outputbeam, while the remainder of the input beam exits the first waveguide asthe transmitted output beam.

The structure shown in FIG. 24 can also be used to couple between TE andTM polarized modes. The electro-optic coefficient r₅₁ enables couplingbetween the two orthogonal polarizations in a lithium niobate crystal,for example. As before, the period of the grating is chosen so that thegrating constant is equal to the difference in propagation constantsbetween the two modes. The interaction length is chosen to optimize thepower transfer.

A waveguide, such as a titanium-indiffused waveguide which supports bothTE and TM modes, is used in applications where both polarizations canenter or leave the converter. A waveguide such as a proton exchangedwaveguide which supports only one polarization (TM in z-cut lithiumniobate substrates or TE in x- or y-cut) can be useful in applicationswhere only a single polarization is desired. Such a one-polarizationwaveguide can act as a very effective filter for the other polarization.The wrong polarization component will rapidly disperse away from thewaveguide due to diffraction, leaving only the guided polarization inthe waveguide. For example, the proton exchanged output waveguide 731may act to guide only the input polarization or only the outputpolarization, as desired. This device can be used as an opticalmodulator with excellent transmission and extinction if the gratingcoupling is strong, and the interaction length and electric field areselected correctly. A modulator configured with a proton exchangedwaveguide will transmit essentially all of the correctly polarized inputlight, and produce very low transmission of light which is coupled intothe perpendicular polarized mode. Alternately, the input waveguide maybe titanium-indiffused to accept either polarization at the input. Theindex profiles that form the waveguides for the two beams are preferablysimilar so that the profiles of the TE and TM modes overlap well, andthe coupling efficiency is maximized.

To activate the r₅₁ coefficient, an electric field is applied along theY or the X axis of the crystal. The electrode configuration that willachieve the appropriate field direction depends on the cut of thecrystal. For a z-cut crystal with a waveguide oriented along the x axis,the first electrode and second electrode can be placed on either side ofthe waveguide. Alternately, for a y-cut crystal with a waveguideoriented along the x axis, the first electrode can be placed directlyover the waveguide, with the second electrode on either side of thewaveguide, parallel to the first electrode.

Since the poled domains in the grating 722 can be made to extend througha bulk substrate (such as 0.5 mm thick or more), the structure of FIG.24 is also useful for a controllable bulk polarization converter. Inthis case the waveguide 730 is unnecessary, and the electrodes areoptimally configured on either side of a thin bulk slab of poledmaterial.

Referring to FIG. 25, there is shown a switched beam director 700incorporating a wye power splitter 702 and a transverse mode converter704. The mode converter works in a similar way to the transverse modeconverter described above in relation to FIG. 24. The grating structure706 phase matches energy conversion from the lowest order (symmetric)mode incident in waveguide 708 into the next higher order(antisymmetric) mode of the waveguide. The length and strength of theinteraction region where the waveguide and the grating structure overlapare chosen to convert approximately half of the input single symmetricmode power into a higher order antisymmetric mode. Furthermore, theoptical path length between the grating mode converter section 704 andthe wye splitter 702 is chosen so that the phase of the two modes addsconstructively at one of the branches 712 of the wye and destructivelyat the other branch 713. The result is that the power is routedprimarily into the waveguide 712 with the constructive interference,with very little power leakage into the other waveguide 713. In thiscondition, any reverse propagating power in the guide 713 is essentiallyexcluded from coupling into a reverse propagating mode in the guide 708after the mode coupler 704. The device forms an efficient power routerin the forward direction and an isolating structure in the reversedirection.

By adjusting the optical path length between the grating mode convertersection 704 and the wye splitter 702, it is possible to switch theoutput power from guide 712 to guide 713. This is done by adjusting therelative optical path length for the lowest order mode and the higherorder mode so that the two modes slip phase by π relative to each other,now producing constructive interference in the guide 713 and destructiveinterference in the guide 712. The relative path length adjustment canbe achieved in the path length adjustment section 705 by exciting theelectrode pair 711 and 709 with the voltage source 714, changing theindex of refraction under the electrode 711 via the electro-optic effectin the substrate 703, which is preferably lithium niobate (but may beany electro-optic material with transparency for the waves such aslithium tantalate, KTP, GaAs, InP, AgGaS₂, crystalline quartz, etc.).The propagation distance of the waveguide 708 under the electrode 711 isselected, along with the excitation voltage, to enable changing therelative phase of the two modes by at least the desired amount.

The grating 706 may be a permanent grating fabricated by any of thetechniques known in the art. However, to optimize the functioning of thedevice, it is desirable to have almost exactly equal power in thesymmetric and the antisymmetric modes. It is difficult to achievesufficient control in existing fabrication techniques to achieve thisgoal, and it is therefore desirable to have some adjustment in thegrating strength. This adjustability can be achieved with the use of atleast some poled grating sections, excited by the electrodes 709 and710, which are driven by the power supply 715, and which can be used bythemselves to accomplish the desired mode conversion, or to adjust thestrength of a combined poled-permanent grating.

The input waveguide 708 is best implemented as a single mode waveguideincorporating a (preferably adiabatic) taper 701 to permit guiding ofthe two modes between the transverse mode coupler 704 and the wyesplitter 702. The waveguides 712 and 713 are both preferably singlemode. While any order modes may be used in the device as long as theirsymmetry is opposite, it is most desirable for interconnection purposesto work with the lowest order mode at the input and output legs. Theintermediate excited mode is less critical, and could be, for instance,a higher order antisymmetric mode.

FIG. 26 shows a parallel waveguide switchable resonator 750 in which aninput waveguide 752 is coupled to a parallel waveguide 754 along aninteraction region 753. Grating reflectors 755 and 756 are disposedacross the waveguide 754 in such a way as to retroreflect lightpropagating in the guide. The pair of separated reflectors and thewaveguide 754 form an integrated etalon coupled to the input waveguide752. The length of the coupling region 753 and the separation of theparallel waveguides in the coupling region are chosen so that a certaindesired fraction T of the input beam 757 is coupled into the waveguide754. The light coupled into the etalon structure 754, 755, and 756resonates between the reflectors 755 and 756, and couples out into twoprinicipal output channels: the forward propagating wave 759 and thereverse propagating wave 758 in waveguide 752. The same fraction T ofthe power circulating in the etalon couples into each of the two outputchannels 758 and 759.

As for any etalon, the integrated etalon has a frequency acceptancestructure comprised of multiple peaks in frequency space with widthdependent on the loss of the resonator, and separation equal to the freespectral range. If the optical frequency of the input beam 757 matchesone of these resonant frequencies, the power circulating in the etalonwill build up to a value P_(circ) determined by P_(circ) =P_(inc)T/(T+Γ/2)² where P_(inc) is the incident power 757 in the waveguide 752,Γ is the loss of the etalon not including the output coupling into theforward propagating wave 759 and the reverse propagating wave 758 inwaveguide 752, and we have assumed weak coupling and low loss. Theoutput coupled wave from the etalon which propagates in the reversedirection in waveguide 752 forms the reflected wave 758. The reflectedpower in beam 758 is equal to P_(ref) =P_(inc) (1+Γ/2T)² on the peak ofthe resonance. When T>Γ/2, essentially all of the incident power isreflected. The output coupled wave from the etalon which propagates inthe forward direction in waveguide 752 is out of phase (on a cavityresonance) with the uncoupled portion of the input wave 757, and the twobeams destructively interfere, producing a low amplitude output beam759. Because the two beam have unequal amplitude, the residual powerP_(trans) =P_(inc) (1+2T/Γ)² in the output beam 759 is not quite zero,but it can be very close. If the coupling T is made very large comparedto the loss Γ of the etalon, the transmission of the device is greatlysuppressed (by 26 dB if T=10Γ). This structure then acts as a very lowloss reflector at a comb of frequencies separated by the FSR.

The device can be switched by changing the optical path length betweenthe two reflectors 755 and 756. Electrodes 761 and 762 are disposed toproduce an electric field through the waveguide 754 between the mirrors755 and 756. The electrodes are excited with a voltage source 763,changing the effective index of the substrate under the electrode 761via the electro-optic effect, thereby changing the optical path lengthbetween the mirrors and shifting the resonances of the integratedetalon. If the resonances are shifted by more than either the width ofthe resonances or the frequency bandwidth of the incident beam, thereflection will drop to zero, and the transmission will rise toessentially 100% as the circulating power within the etalon issuppressed to approximately P_(inc) T/4.

The gratings 755 and 756 may be permanent gratings, or they may be poledgratings excited by electrodes as shown in previous diagrams anddiscussed above. If the grating 756 is a poled grating, the device mayalso be switched by switching it off. With grating 756 off, i.e. notreflecting, the loss to the incident wave 757 is equal to the couplingconstant T, but now the comb structure is eliminated instead of justbeing frequency shifted as by the electrode 761. The difference inswitching function between these two modes of operation may besignificant with for example a broadband input signal where it isnecessary to switch off the reflection rather than just change itsfrequency. For a single frequency input beam, the reflection can beswitched equally well by changing the path length with electrode 761 orby spoiling the Q of the resonator by switching off the mirror 756.However, if the reflectivity of the mirror 756 is retained and only thefrequency spectrum of the etalon is shifted with the electrode 761,other frequency components of a broadband input wave would be reflected,and this might be highly undesirable in some applications.

The power P_(circ) which builds up in the etalon can be quite large if Tand F are small, and can be useful in applications such as secondharmonic generation, for example. In this application, aquasi-phasematched (QPM) periodic poled structure in a section of thelithium niobate substrate is incorporated into the resonator between,say, the mirror 756 and the interaction region 753, or possibly withinthe interaction region itself. One of the resonant frequencies of theetalon is then tuned to coincide with the phasematching frequency forthe QPM frequency doubler. The power buildup which occurs enhances thefrequency conversion efficiency of the device as the square of thebuildup factor P_(circ) /P_(inc). The high reflection which occurs atthis frequency can also be used to injection lock the pump laser to thedesired frequency if the FSR is large enough that the other resonantmodes are not injection locked simultaneously. The linear integratedetalon geometry described above in reference to FIGS. 21 and 22 can alsobe used to accomplish the same purposes.

To optimize the power building up in the etalon between the reflectors755 and 756, the losses in the resonator must be minimized. The couplingof FIG. 26 cannot be "impedance matched", in analogy to the processknown in the art of bulk buildup cavities, where the input coupling intothe resonator is adjusted to cancel by destructive interference theportion of the incident beam which is not coupled into the cavity. Thisis the condition of the etalon transmission interference peak. Asdescribed above, what happens in the integrated structure is that thetransmitted beam can be nearly cancelled while the power builds up inthe coupled resonator, but a strong reflected wave emerges. Thereflected wave may be eliminated in a ring waveguide structure, as isillustrated in FIGS. 27 and 28.

An output 751 proportional to the power circulating within the etalonmay be taken through the grating 756, if desired, or alternately throughthe grating 755.

In FIG. 27, a three-arm etalon 760 is shown with an input waveguide 752,a parallel waveguide coupling region 753, a ring resonator formed bythree waveguide segments 764, 765, and 766, three grating reflectors767, 768, and 769. The optical path length adjustment section formedbetween the electrodes 761 and 762 is optional. The grating reflector767 is disposed to optimally reflect the power arriving from waveguide764 into the waveguide 765. In a single mode system, the spatialconfiguration of the grating (and its electrodes if any) is designed tocouple from the lowest order mode of waveguide 764 into the lowest ordermode of 765. The gratings 768 and 769 are similarly configured tooptimize the power flow from waveguide 765 into waveguide 766, and theninto waveguide 764 again, forming a Fabry-Perot resonator with adeterminate optical path length, FSR, optical loss coefficient, andcoupling T with the input waveguide 752. Now, impedance matching ispossible, and is accomplished when the coupling coefficient T equals thetotal round-trip loss coefficient of the resonator less the outputcoupling loss, principally in the coupling region 753. If a phasematched frequency doubler is disposed within the resonator, theconverted power out of the fundamental frequency beam circulating in theresonator does count as one of the losses in the total round-trip loss.

If an input beam 757 is incident on the device with a frequency equal toone of the resonances of the three-arm etalon, power will couple acrossthe parallel waveguide interaction region into the etalon and build upto a circulating power of P_(circ) =P_(inc) T/(T+Γ)². Because of thering structure, the power will circulate primarily in one direction,from waveguide 764 to waveguides 765, 766, and back to 764. There is nowonly a single output coupled wave from the etalon onto the waveguide752, and it propagates in the forward direction. The output coupled waveinterferes destructively with the remainder of the input wave 757,forming a weak transmitted wave 759. The transmitted power P_(trans) inthe output beam 759 is given by P_(trans) =P_(inc) (1-Γ/T)² /(1+Γ/T)²,and can be brought to zero if Γ=T, which is the impedance matchedcondition. In this case, all the incident power flows into theresonator. In the impedance matched condition, the two beams have equalamplitude, and the transmitted power drops to zero. There is essentiallyno reflected power in beam 758 except for reflections fromdiscontinuities in the waveguide 752, which can be minimized by gooddesign.

The grating 767 or any of the other gratings may be configured as aswitchable grating, in which case the quality Q of the etalon may bespoiled by turning off the grating, eliminating the comb structureentirely but leaving some optical loss due to power coupled into thewaveguide 764. An output beam 751 may be taken in transmission throughthe grating 768, and/or through the gratings 767 or 769.

FIG. 28 shows a ring waveguide etalon 770. As before, the inputwaveguide 752 is coupled to a waveguide 772 in a parallel interactionregion 753. The interaction region 753 includes a grating in FIG. 28(although it is not required) to emphasize that grating coupling is auseful option in the etalon geometry of FIGS. 26, 27, and 28. Thewaveguide 772 follows a curved closed path (with any geometry includingpotentially multiple loops with crossings), feeding a portion of thepower emerging from section 753 back into the interaction region 753. Asbefore, electrodes 761 and 773 are supplied to allow the optical pathlength, and hence the FSR to be adjusted, although in this case they areshown disposed on the same face of the substrate. A straight section 771is provided where certain critical functional components may befabricated, according to the application of the etalon structure. If theetalon device 770 is used for frequency doubling, it would beadvantageous to insert the frequency doubling structure into a straightsection such as 771 of the ring, but provision must be made to couplethe frequency converted light out of the ring waveguide.

The functioning of the device 770 is otherwise similar to that of thedevice 760. While the device 760 may consume less surface area on asubstrate, the device 770 may have lower optical loss in the etalon,particularly if the diameter is one cm or larger.

The devices 760 and 770 can function as buildup cavities for frequencydoubling in which the feedback into the optical source is minimal. Theycan also switch the transmission of a given frequency withoutretroreflection, which is useful in applications including opticalcommunications.

In WDM communications, many communications channels separated by theiroptical wavelength may be carried on the same optical fiber. To detect achannel, the light in the desired wavelength region must first beseparated from the remaining channels which are routed to otherdestinations. This separation function is performed by a channeldropping filter. A channel dropping filter is a communications devicewhich is used in a wavelength division multiplexed (WDM) environment. Itis desired to multiplex several channels across a single transmissionfiber by carrying the channels on different wavelengths. A criticalcomponent in such a system is a channel dropping filter which allows theextraction of a single channel for routing or detection purposes. Theideal filter will extract essentially all of the light in a channel withgood extinction ratio, so that the same wavelength may be used later inthe network without undesirable crosstalk. It must have very lowinsertion losses for the out-of-band components because multiple channeldropping filters may be installed on any given line. Preferably, itshould be switchable so that a channel may be dropped at a destinationlocation, and after the communication is finished, the channel maycontinue past that location to another destination. The inverse of thechannel dropping filter is the channel adding filter which adds achannel to a fiber without significantly affecting the power propagatingin the other channels. Transmission and reflection filters have beenanalyzed in detail [HL91, KHO87]. Several of the above structures may beused for channel dropping filters, including the devices described inreference to FIGS. 7, 10, 26, 27 and 28.

The grating coupled waveguide tee of FIG. 7 is a channel dropping filterwith low loss for the out-of-band components. With prior art gratings,this configuration has difficulty with crosstalk, since achieving 99.9%outcoupling for the in-band component requires a very long grating. Thecoupling strength of our periodic poled gratings is significantlyincreased over the prior art, due to the ability to use higher ordergratings with sharp interfaces which extend entirely across thewaveguide. Whereas the prior art is limited to shallow waveguides tooptimize the overlap between the necessarily shallow grating and thewaveguide, we are able to use the lower loss waveguide configurationwith essentially equal depth and width because our grating structureextends entirely across the depth of the waveguide. This structure canalso be used as a channel adding filter.

The device of FIG. 10, if the grating is configured as described in Hauset al. "Narrow band optical channel dropping filters" J. LightwaveTechnol. 10, 57-62 (1992), is also a channel dropping filter. Ourcontribution in this case is only the poled grating coupling technique,which enables strong coupling between the waveguides in a shortdistance, and which relieves fabrication difficulties in permittingefficient higher order gratings to be produced.

The devices 750, 760 and 770 can be used as channel dropping filters bytuning a resonance of the etalon to the frequency of the channel to beextracted from the input waveguide 752. If the integrated etalon isnearly impedance matched, essentially all the power at the resonantfrequency is transferred into the etalon. In the ring geometries ofFIGS. 27 and 28, the transmitted and reflected powers in the waveguide752 can be reduced to any desired level, minimizing crosstalk. The lightcorresponding to the desired channel is completely extracted (dropped)from the input waveguide, leaving neither reflections or transmissions.In the linear geometry of FIG. 26, some light is lost to reflection,which does not significantly reduce the detection efficiency, but whichmay cause crosstalk problems in a communications network. The signalcarried by the light can be detected by placing a detector over awaveguide segment of the etalon and coupled to the light in thewaveguide. Or, the detector can be coupled to one of the outputwaveguides such as 754 in FIG. 26, 764, 765, or 766 in FIG. 27, and 794in FIG. 28. In the case of the device 760, the outcoupling can beaccomplished by adjusting the reflection of one of the resonator gratingreflectors 767, 768 or 769 so that a small portion of the circulatingpower is coupled out into the continuation segments of the waveguides asshown for output beam 751. Those continuation waveguide segments mayalso be connected to ports of other devices, which may be eitherdiscrete devices or integrated on the same substrate. In the case of thedevice 770, a parallel waveguide output coupler (with or withoutgrating) may be placed in the straight section 771 of the ring. Althoughonly a fraction of the circulating power may be outcoupled at theseports, the total outcoupled power may be very close to 100% of thochannel power entering the waveguide 752 due to the buildup which occursin the etalon. Output coupling is shown with an adjacent waveguide 794,producing the output beam 751.

The ring geometries excel in terms of extinction ratio (which is highwhen the light separation efficiency is high) and low crosstalk becausethey can be adjusted to have almost total transfer of power into theetalon. All of the etalon devices can be designed with very lowinsertion loss for the out-of-band beams. All of the devices of FIGS.26-28 are switchable by means of the phase shifting electrodes 761, and762 (and 763 in FIG. 28).

As described before, the optical path length may be adjusted usingelectrode 761 to shift the frequency of the integrated etalonresonances. The desired channel may be selected this way directly. Or,multiple channels may be selected by this technique using the approachdescribed above in reference to FIGS. 20, 21, and 22; if the FSR of theetalon is selected to be slightly different from the channel separation,the Moire effect is used to select widely spaced channels with a minimumof continuous tuning. (A good choice is to make the FSR equal to thechannel spacing plus a few times the frequency width obtained whenconvolving the channel bandwidth with the etalon resonance bandwidth).

As a variation on the structures 750, 760, and 770, the coupling region753 may be implemented as a grating-assisted coupler as described abovein reference to FIG. 10. This has the advantage, in the poled-gratingimplementation, that the coupling fraction T can be adjusted.Particularly for the ring resonator designs 760 and 770, an adjustablecoupling is useful to achieve impedance matching. As a furthervariation, the electrodes may be implemented on the same face of thesubstrate, as described above to obtain lower voltage excitation.

The structures of FIGS. 27 and 28 may also be used as efficient channeladding filters if the signal to be added to the output beam 759 isbrought in on the waveguide 766, for example, or if it is coupled intothe straight section 771 via the waveguide 794. These input interactionswill preferably be impedance matched.

Referring now to FIG. 29A, there is shown a waveguidemodulator/attenuator 800 using a poled segment 806. The function of thepoled segment 806 is to (switchably) collect the light emitted from aninput waveguide segment 802 and launch it into an output waveguidesegment 804 when switched on. In this device, an input light beam 820 iscoupled into the input waveguide 802. A poled segment 806 is positionedbetween the input segment and the output waveguide segment 804. Theinput and output waveguide segments are preferably permanent waveguideswhich may be fabricated by any of the standard techniques includingindiffusion and ion exchange. The segment 806 is preferably a reversepoled region within a uniformly poled substrate so that there isessentially no difference in index of refraction and hence nowaveguiding effect when the electric field is off. The segment 806 is awaveguide segment as shown in FIG. 29A. (It may alternatively beconfigured in several geometrically different ways such as a positivelens structure, a negative lens structure, or a compound structure forrelaying light between many such elements: see FIG. 29B.) The segment806 is turned on by applying an electric field through the segment. Theelectric field changes the index of refraction of the poled segment andsurrounding regions. Because the segment 806 is poled differently(preferably reverse poled) from the substrate material, the index of thesegment can be raised relative to the surrounding material by applyingthe correct field polarity, forming a waveguide. The index inside theboundary of the waveguide may be increased, or the index at and outsidethe boundary may be depressed. When the poled segment is on, acontinuous waveguide is formed, joining the input and output segments.This is achieved by butting the waveguides together, aligning them tothe same axis, and adjusting the width of the poled segment so that itstransverse mode profile optimally matches the mode profile of the inputand output waveguides 802 and 804.

With the poled segment off, the input beam is not confined in the poledregion, so that the beam expands substantially by diffraction before itgets to the output waveguide segment. If the separation of the input andoutput waveguide segments is much greater than the Rayleigh range of theunguided beam, so that the beam expands to a dimension much larger thanthat of the output waveguide, only a small portion of the input beamwill be coupled into the output waveguide segment to form the outputbeam 822. By adjusting the length of the segment 806 relative to theRayleigh range, the amount of power transmitted in the off condition canbe reduced to the desired degree.

The location of the ends of the poled segment 806 are adjusted relativeto the locations of the ends of the input and output waveguides tominimize the loss caused by the discontinuity. Because the permanentwaveguides have a diffuse boundary, the poled waveguide has a discreteboundary, and the index change in the switched segment adds to thepre-existing index, it is desirable to leave a small gap on the order ofhalf the diffusion length between the lithographically defined boundaryof the waveguides 802 and 804, and the ends of the poled segment 806. Tofurther reduce the reflection and other loss at the junction betweenwaveguides 802 and 806, it is also advantageous to taper the onset ofthe index change in the segment 806 by either making the excitingelectrode 810 slightly shorter than the segment 806 or by tapering theelectrode width near its end, in both cases taking advantage of thereduction of the electric field by the fringing effect.

One distinguishing aspect of this configuration is that the reflectedpower can be minimized in both the on and the off conditions. With theswitch off, the reflection is dominated by the residual reflection atthe end 803 of the waveguide 802. This reflection may be minimized bytapering the reduction of the index difference along the length of thewaveguide. The reflection from the end 805 of the waveguide 804 issuppressed by the square of the "off" transmission. In the "on"condition, the reflection is minimized by also tapering the indexdifference of the structure 806 along the direction of propagation,creating a smooth boundary rather than a sharp interface.

The boundaries of the excited poled region confine the beam laterallywhen they are activated because of the increase in the index ofrefraction within the boundaries. If the depth of the poled regionequals the depth of the waveguides 802 and 804, the beam is alsoconfined in the vertical direction by the poled segment boundaries.However, it is difficult to control the depth of the poling in a z-cutlithium niobate wafer. It is easiest to pole a deep domain, and take oneof several alternative measures to obtain confinement in the verticaldimension. The preferred approach is to arrange the electrodes so thatthe amplitude of the electric field falls off in the vertical dimension.This is achieved by the same-side electrode configuration shown in FIG.29A, but not with electrodes placed on opposite sides of the substrate.The penetration depth of the electric fields can be reduced by narrowingthe gap between the two electrodes and by reducing the width of theoverall electrode structure.

In addition or as an alternative, a weak permanent waveguide can befabricated in the volume between the input and output waveguides, whichis insufficient to convey much energy by itself, but which incombination with the index elevation produced in the poled segment 806can optimally confine the light in two dimensions to convey essentiallyall the light into the output waveguide 804. This can be done, forexample, by adjusting the permanent index change (relative to thesubstrate) within the segment to about 0.6 of the index change in thewaveguides 802 and 804. If the "on" index change in the segment 806 isadjusted to about 0.5 of the same value, the combined index change issufficient to achieve reasonable guiding while the permanent indexchange is insufficient. In the "on" condition, the mode is confined inboth transverse dimensions even though the switched index changeproduced in the poled region may be considerably deeper than the desiredwaveguide dimension: the effective depth of the "on" waveguide is mainlydetermined by the permanent index change. The weak waveguide may befabricated in a second masking step, or it may be fabricated in the samemasking step with a narrower mask segment defining the weaker waveguidesegment.

As a related alternative, the region between the input and outputwaveguides may be a planar waveguide, in which case the propagating modecan at minimum diffract in one dimension. Switching on a poled sectionwill in this case add the needed transverse confinement despite having adeeper index change than the planar waveguide. Since in both cases theconfinement of the waveguide in the two dimensions is achieved by twoindependent techniques, switchable waveguides of essentially any aspectratio (the ratio of the waveguide width to depth) can be formed. Boththe planar and channel waveguides can be fabricated by the sametechnique, which is preferably the annealed proton exchange process.Separate proton exchange steps may be used to define the planar guideand channel waveguide. The waveguide fabrication process is completed byannealing, during which the index changes are diffused down to thedesired depth, and the optical activity of the material is restored.Preferably, the two sets of guides are annealed for the same length oftime, although one set can be made deeper by partially annealing beforethe second proton exchange step is performed.

An important alternative is to use a full, uniform permanent waveguidetraversing the poled segment 806, and to use the electrically excitedsegment to turn off the guiding. In this case, the polarity of the fieldis chosen to depress the index in the poled region, and the depth of thepoled region can be very large (in fact this has some advantage in termsof mode dispersal). This type of switched waveguide is normally on (i.e.transmitting), and requires the application of an electric field toswitch it off. There are advantages to both normally-on and normally-offswitch configurations in terms of their behavior during a power failure,so it is important that this invention is capable of providing bothmodes. To switch the waveguiding off in the segment 806, an index changeis desired which is approximately equal and opposite to the index changeinduced in the permanent waveguide. The effect of the variation withdepth of the electric field on the "off" state is quite small because itis sufficient to suppress the majority of the waveguide in order tostrongly disperse the light.

Confinement can be achieved in both dimensions without the need of aplanar waveguide, by a finite-depth poling technique. Several polingtechniques (such as for example titanium-indiffusion in lithium niobateand lithium tantalate and ion exchange in KTP), produce poling to afinite depth, which can potentially be optimized to form a poled channelwaveguide with a particular depth. These techniques, however, produce anindex change along with the poling, forming somewhat of a permanentwaveguide depending on the processing parameters. Depending on thestrength of this index change, the poled waveguide segment may befabricated in either the "normally on" or the "normally off"configuration.

Preferably, the electric field is created in the poled region byapplying a voltage across two electrodes, which are laid out on the sameface of the crystal as the polled waveguide segment. A first electrode810 is laid out over the poled region, while the second electrode 812 isplaced in proximity to one or more sides of the first electrode. For az-cut crystal, this configuration activates the d₃₃ electro-opticcoefficient of the substrate. A voltage source 816 is electricallyconnected via two wires 814 to the electrodes to provide the drivingvoltage for the device. This device can be used as a digital ornonlinear analog modulator. A full-on voltage is defined to be thevoltage at which the loss across the poled region is the lowest. The offvoltage is defined as that voltage which reduces the coupling to theoutput waveguide segment to the desired extent. By continuously varyingthe voltage between the on and the off voltages, the device can be usedas either an analog modulator or a variable attenuator.

In an alternative structure, the structure 806 forms a switched curvedwaveguide, which again aligns with the input 802 and output 804waveguides. The mode of such a structure is called a "whisperinggallery" mode in the extreme case where the curvature is small and themode confinement on the inside edge becomes independent of the insidewaveguide edge. For larger curvatures, the mode is a modified whisperinggallery mode where some confinement is provided by the inside edge ofthe waveguide. The poled structure provides an advantage in addition tothe switchability, namely that the sharp index of refraction transitionon its outside wall greatly improves the confinement of the modifiedwhispering gallery mode which propagates in the curved waveguide. Theinput and output waveguides need not be coaxial or parallel in thiscase, potentially increasing the forward isolation in the switched-offcondition. If the input and output waveguides are arranged along axes atan angle to each other, the structure 806 may be a curved waveguidesegment with a single radius of curvature or a tapered radius ofcurvature, used to optimally couple power between them when the curvedwaveguide structure 806 is turned on.

FIG. 29B shows an alternative structure 801 which is a switched lensmodulator/attenuator in which the prismatic structure of segment 806 ismodified into a lenslike structure in which the product of the localoptical path length and the local (signed) index change is reducedquadratically with transverse distance away from the axis of the guides802 and 804. The lenslike structure is placed such that it concentratesor refocuses the beam 821 emerging from the end 803 of the inputwaveguide 802 into the end 805 of the output waveguide 804. The opticalwave is allowed to diffract away from the end 803, and passes throughthe lenslike structure 807. Note that in this structure, multipleelements may be placed adjacent each other, increasing the net focussingeffect. The index of refraction within the regions 807 is increased toobtain a focussing effect. If the surrounding region is poled in areverse direction to the regions 807, or if the electro-opticcoefficient of the surrounding region is otherwise opposite to that ofthe regions 807, the spaces between the lenses also act as focussingregions. (The negative lens shape formed by the regions between thelenses 807, excited to a lower index value, acts as a converging lensstructure.) The electrode 810 is placed over the structure 806 withelectrodes 812 being placed outside the structure but adjacent theelectrode 810 with a gap as desired. When the electrodes are notactuated, the beam continues to diverge, and very little power isrefocussed into the waveguide end 805. When the switch is on, the beamis refocussed, and a fraction of the power continues through the guide804. Vertical confinement is needed for efficient power collection inthe on state, while it is undesirable in the off state. Verticalconfinement may be provided as needed by, for example, providing auniform planar waveguide 835 across the entire surface on which thestructures are patterned. Vertical confinement may also be provided bythe lenslike structure 806 if it is poled deep into the substrate, andthe electric field reduction as a function of depth is tailored tocollect and refocus the energy back to the waveguide end 805. Thestructure of FIG. 29B may of course also be used in other contexts whichmay not have one or both waveguides 802 and 804.

Referring to FIG. 30, there is shown a poled total internal reflecting(TIR) optical energy redirector 830 using a poled waveguide segment.This figure illustrates both a poled TIR reflector for high switchedreflection combined with a poled waveguide segment for low insertionloss. An input waveguide 832 extends entirely across the device. A poledregion 836 extends across the waveguide at an angle 848, forming a TIRinterface for the beam propagating in the guide when the poled region iselectro-optically activated. A portion of the poled region also forms apoled waveguide segment 837 that is connected to an output waveguidesegment 834. The poled waveguide segment and the output waveguidesegment are both laid out at twice the angle 848 with respect to theinput waveguide. A voltage source 846 provides the electrical activationfor the switch, and is connected to it through two wires 844.

The poled region 836 is defined by six vertical faces according to thediagram, with one face traversing the waveguide 832 at a shallow angle848 equal to the TIR angle and less than the critical angle for totalinternal reflection for a desired electrode excitation. This face is theTIR reflecting interface. The next three consecutive vertical faces ofthe poled region enclose a projection outside of the waveguide 832. Theprojection is a switchable waveguide segment. The placement of the nexttwo vertical faces is not critical, and may follow the waveguideboundaries and cross it at 90°.

The domains (836 and the region of the substrate outside 836) arecharacterized by a quiescent index of refraction distribution, which isthe spatial distribution of the index in the absence of applied electricfield. When an exciting electric field distribution is applied throughthe domains, they will have an excited index of refraction distributionwhich is different from the respective quiescent distribution. Theexcited distribution will also have a range according to the accessiblerange of the applied electric field. The advantage of juxtaposing twodomain types near one another is that the electric response may beopposite in the two domains, producing a transition with double thechange in index across the region of juxtaposition. In the case of indexor refraction changes, the transition forms a reflection boundary withlarger reflection than would be attained with a single domain type.

When the switch is on, an input beam 851 that is coupled into thewaveguide reflects off the TIR interface, propagates down the poledwaveguide segment, and passes into the output waveguide segment 834 toform a deflected output beam 854. When the switch is off, the input beampropagates through the poled interface and continues through the inputwaveguide to form an undeflected output beam 852. Because the indexchange at the TIR interface is low, the reflection in the off state isvery low. Because the permanent waveguide segment 834 is separated byseveral mode exponential decay lengths from the guide 832, the powerlost due to scatter as the beam passes by the switching region is alsoextremely low. An "off" switch is essentially invisible to thewaveguide, producing extremely low loss in the input guide. Theadditional loss of the switched region in the off state compared to anequal length of unperturbed waveguide is called the insertion loss. Lowinsertion loss is especially desirable when the input waveguide is a buswith many poled switches.

The angle θ (848) of the poled interface with respect to the inputwaveguide must be less than the maximum or critical TIR angle θ_(c), asderived from Snell's law: ##EQU3## where θ=TIR angle (between thewaveguide and the poling interface),

n=index of refraction of waveguiding region, and

Δn=electro-optic change in index on each side of poling boundary

Since the index change occurs on each side of the poling boundary withopposite sign, the effective index change is 2Δn. This expressionassumes slowly varying (adiabatic) changes in the index away from theboundary. Due to the doubling in the effective index change, the maximumswitching angle that can be achieved with a poled TIR switch isincreased by √2 over the prior art switches with a pair of electrodesand no poled interface. This is a very significant increase since itincreases the maximum packing density of switch arrays which can beachieved using a TIR switch.

The critical angle θ_(c) depends on the polarization of the input beambecause the index change Δn depends on the polarization. In z-cutlithium niobate, for example, with a vertical field E₃, the TM wave issensitive to the change in the extraordinary index of refraction throughthe r₃₃ and the TE wave to the change in the ordinary index through r₁₃.Since r₃₃ >r₁₃, it is far easier to switch TM waves. Use of annealedproton exchanged waveguides is very convenient because they guide onlywaves polarized in the z-direction. In x-cut y-propagating (or y-cutx-propagating) lithium niobate, on the other hand, the TE wave has thehigher change in index. Note that in this case, the electrodeconfiguration must be changed to produce a field component in the zdirection in the plane of the substrate, instead of in the verticaldirection.

The design angle for actual TIR switches must be chosen after optimizingseveral factors. The mode to be switched includes two angulardistributions (in the waveguide fabrication plane and out of the plane)which can be different if the widths of the waveguide in the two planesare different. The angular content δφ of the mode in a given planecovers approximately δφ=±λ/πw_(o) where w_(o) is the 1/e² mode waist inthat plane. We wish most of the light to be reflected at the TIRinterface, so the angle of incidence must be less than the criticalangle θ_(c) by approximately the angular content δφ in the plane of theswitched waveguides. The angular content δφ is inversely related to thewaist size, but so is the packing density which we wish to optimize. Theangular content of the mode in the direction out of the plane of thewaveguides also must be taken into account because it also contributesto the effective incidence angle, although in a geometrically morecomplex way.

An alternative way of producing a TIR switch is with a strain fieldinstead of or in addition to the electric field. The strain field ismost conveniently implemented in a permanent fashion; the electric fieldis most useful for producing changes in the reflection. An orientedstrain field applied at a domain boundary produces different changes inthe index of refraction, via the photoelastic effect, in the twodomains, resulting in an index of refraction interface. As mentionedabove in reference to FIG. 2, the strain field may be produced byheating the sample to a high temperature, depositing a film with adifferent coefficient of thermal expansion, and cooling to roomtemperature. A pattern applied to the film by etching away regions suchas strips will produce a strain field about the gap in the film. Thisstrain field can then be used to actuate an index of refractiondifference at domain boundaries. If the applied film is a dielectric anelectric field may be applied through it to the poled regions providedthat the deposition of electrodes does not undesirably change the strainfield. The film is preferably a film with low optical absorption so thatit can be contacted directly to the substrate instead of being spaced bya buffer layer.

The poled region includes a portion of the input waveguide and has aninterface normal to the propagation axis of the waveguide. The portionof the input waveguide that contains the TIR interface crossing definesthe length of the switch:

where θ is previously defined, ##EQU4## L=length of the switch measuredalong the input waveguide, and W=width of the waveguide

Thus, in order to minimize the size of the switch, the width of thewaveguide must be made as small as possible. For space-criticalapplications, it is preferable that the waveguide segments be singlemode. As a numerical example, if the width of the single-mode waveguideis 4 μm, the maximum index change Δn is 0.0015, and the index ofrefraction is 2.16, then the TIR angle θ is 3° and the length of theswitch L is 76 μm.

The poled waveguide segment forms an angle with respect to the inputguide equal to 2θ, which is the deflection angle of the TIR interface.In order to efficiently modematch the beam reflecting off the TIRinterface into the poled waveguide segment, the poled segment shouldhave nearly the same transverse mode profile as the input waveguide.Efficient mode matching can be achieved by selecting the propercombination of width and index difference of the poled waveguide. Thepoled waveguide segment intersects the input waveguide along the latterhalf of the side of the waveguide occupied by the switch interface. Theexact dimensions and placement of the waveguide are determined tooptimally match the near field mode profile emerging from the totalinternal reflection process to the mode of the waveguide in terms ofdirection of propagation and transverse profile. The same is true of thematch between the poled waveguide segment and the permanent waveguidesegment 834, similarly to what was described above in reference to FIG.29A.

The permanent waveguide segment is essentially a continuation of thepoled waveguide segment. The length of the poled segment depends onoptimizing losses in the input waveguide and the switched waveguide. Inorder to avoid scattering interaction between the undeflected beam inthe input waveguide when the switch is off, the permanent waveguidesegment must be separated by some distance (at least an opticalwavelength) from the input guide. For a bus waveguide with manyswitches, the loss in the input guide must be reduced to a value relatedto the inverse of the number of switches. The modal profile of a beam inthe input guide extends a certain distance beyond the indiffused edge ofthe guide, where it decays exponentially. If the permanent segment isseparated from the input guide by several of these exponential decayconstants, the loss can be reduced to an acceptable level for a buswaveguide.

The length of the poled segment affects the loss in the reflected beamas well. The poled waveguide segment may have higher losses per unitlength than an indiffused waveguide, due to higher wall roughness. Inaddition, there are the above mentioned mode conversion losses at eachend of the waveguide, which are minimized by optimally matching the modeprofiles. If the poled segment is short (on the order of the Rayleighrange of the beam), the transmitting beam does not substantially convertinto the mode of the poled segment, thus reducing the coupling losses.The optimal length of the poled segment depends on the relative lossthat is tolerable in beams in the input waveguide and the switchedwaveguide.

As in the case of the waveguide segment modulator/attenuator shown inFIG. 29A, there is a need for vertical confinement of the mode in theswitched waveguide segment 837. The same options described there can beimplemented here. Shown in FIG. 30 is a planar waveguide 835 whichconfines the beam in a plane parallel to the surface of the substrate.Since the planar waveguide is uniform, its presence does not affect theloss of the waveguide switch junction in its off state. In place of theplanar waveguide, or in some combination, the other alternatives mayalso be implemented, including tailoring the depth of the electricfields to obtain vertical confinement, using short depth poling, using apartial waveguide which is augmented by the field induced index change,and using a full permanent waveguide which is turned off by a fieldactivated poled region. The latter two alternatives have thedisadvantage that the loss to the beam through waveguide 832 is higherdue to the adjacent index discontinuities.

Horizontal confinement is also an issue in optimizing the switchingregion. If high switched efficiency is desired, it is preferable to havea large TIR reflection angle. The left half of the input wave 851reflects first off the interface 838, forming the right half of thereflected wave. However, after reflection, the right half of thereflected wave is unconfined in the transverse dimension until itarrives into the waveguide segment 837. During its unconfined passage,it will expand by diffraction, reducing the fraction of the beam powerwhich couples into the output waveguide 834. This effect degrades theefficiency of the switch in its on position. However, the mean unguideddistance is limited to approximately the waveguide width divided by fourtimes the sine of the angle 848. The right half of the input waveremains confined after it passes the waveguide segment 837 until itsreflection off the interface 838 because of the permanent index changedue to the right hand side of the waveguide 832. It then matches wellinto the output waveguide 834. Both portions of the input beam 851suffer an undesired reflection from the side of the waveguide 832 afterreflecting from the TIR surface 838. Since this surface is at the sameangle to the axis of propagation of the beam as the surface 838 was, butwith only a fraction of the index difference, there only be a partial,not a total, reflection from this surface which also adds to the loss ofthe switch.

The electrode design is a critical aspect of this switch, in order tooptimize the efficiency of the reflector and minimize the loss of thewaveguide. Preferably, two electrodes are used to activate the switch. Afirst electrode 840 is placed over the TIR interface 838, while a secondelectrode 842 is placed alongside the first electrode, adjacent to thatinterface. The main parameters for optimization are the separation ofthe two electrodes and the distance between the edge of the firstelectrode and the poling boundary, which may or may not overlap. Thespacing between the two electrodes influences the voltage required toactivate the device, as well as the width of the electric field patternwhich penetrates the substrate and produces the index change profile.Electrodes that are spaced further apart require higher voltages, butcreate an electric field that extends deeper into the substrate thanclosely spaced electrodes.

The electric field penetration depth is critical to obtaining a largenet reflection. Because the fields get weaker the farther they are awayfrom the electrodes, the induced index change at the poling boundaryalso drops with depth, as does the TIR angle. At a certain depth calledthe effective depth, the index change becomes insufficient to maintaintotal reflection for the central ray of the optical beam at the angle ofthe switch structure. Since the reflection drops rapidly with indexchange at values below the minimum TIR value, the TIR mirror essentiallystops functioning at this depth. For high net reflection into the guides837 and 834, the device design should be adjusted to create an effectivedepth below the majority of the field profile in the guide 832.

The second important operating parameter influenced by the electrodedesign is the penetration of the evanescent fields of the reflectingwave beyond the TIR interface 838. Although no power may be transmittedbeyond the TIR interface in the "on" condition, the electromagneticfields penetrate the TIR surface by a distance on the order of awavelength. There will also be spatial dependence of the appliedelectric field beyond the TIR surface, the field strength being reduced(and in fact inverted) in regions closer to the other electrode 842. Theindex change is therefore reduced beyond the TIR interface. Care must betaken that the evanescent fields decay to a negligible value beforesubstantial variation in the field occurs, or power will leak throughthe TIR interface. Optimally, the first electrode will overlap thepoling interface by a distance chosen for maximum index change and forsufficient constancy of field beyond the interface 838.

The first electrode also extends across the poled waveguide segment 836,and possibly into adjacent areas. The shapes of the two electrodesexciting this region 836 are determined by optimizing the power flowthrough the waveguide segment and into the permanent waveguide 834.Other electrode structures can be used to modify the strength of theelectric field in the poled region. If, for example, the secondelectrode is extended around the first to form a U shape, the electricfield under the first electrode is increased on the average, but itforms somewhat of a two-lobed waveguide, which may not provide an idealindex profile.

The TIR switch is an optical energy router and can also be used as amodulator. If the voltage source is continuously variable, then themodulator is analog, with a nonlinear relationship between voltage andreflectivity. As the applied voltage is increased, the depth of thetotal reflecting interface is increased, producing a continuouslyadjustable reflection out of the wave 851 into the wave 854. Themodulator can be used in reflection or transmission mode, depending onwhether the transmission should go to zero or 100% when the voltage isremoved. For special nonlinear applications, the nonlinearity of thereflection and transmission coefficients as a function of voltage, suchas where the receiver is logarithmically sensitive to the level of thesignal, might be useful.

FIG. 31 shows a TIR switch with two TIR reflectors. If it is desired toincrease the angle between the output waveguide 834 and the inputwaveguide 832, a second TIR interface 839 may be added. The anglebetween the input waveguide 832 and the output waveguide 834 is doubledrelative to that of FIG. 30, and may be doubled again and again byadding additional TIR interfaces. The interface 839 is created at anangle 849 relative to the interface 838 equal to twice the angle 848.(Subsequent TIR interfaces, if any are added, should be added at thesame angle 838 relative to the previous TIR interface.) The switchedwaveguide portion 837 of FIG. 30 is no longer required since the dualTIR mirror structure brings the light so far away from the inputwaveguide 832 that the permanent waveguide 834 may be butted directlyagainst the end of the poled region 836 without contributing significantloss to the waveguide 832. Again, vertical confinement is provided inthe poled segment 836. The poled segment 836 and the output waveguide834 are configured and aligned so that the field profile propagating inthe chain of TIR and waveguide segments optimally matches the locallowest order mode field profile of the input waveguide 832. After theTIR reflectors, the deflected beam is matched into a permanent waveguide834 to form the output beam 854 when the switch is on.

The shape of the inside boundary of the poled region outside the inputwaveguide 832 is defined by the reflection of the input waveguidethrough the TIR mirrors, one after the other. This definition of theinside boundary achieves optimum guiding of the inside edge of thewaveguide mode while it is reflecting from the two TIR mirrors.

FIG. 32 shows a TIR switched beam director with a poled TIR switch 831with an electrically switched waveguide segment. In this structure, theregion 836 is reverse poled, lies behind the interface 838, and isexcited as before by a pair of electrodes 840 and 842, which areactivated by voltage source 846 and connected via conductors 844. Thepolarity of the excitation is again selected to produce a negative indexchange coming from the direction of the input beam 851. When the switchis on, the beam is reflected off the TIR interface 838 towards thepermanent waveguide 834, but unlike in FIG. 30, there is no poledwaveguide segment joining them. Instead, the electrode 842 is extendedover the intermediate region between the input waveguide and the outputwaveguide 834. A coupling waveguide segment may be formed by applying anelectric field to a region between a lateral boundary of the segment ofthe input waveguide 832 containing the TIR reflection boundary and aninput boundary of the output waveguide 834. The three dimensionaldistribution of the electric fields is determined, as always, by theshape of the electrodes and Maxwell's equations. The electric fieldsproduced by that electrode produce a positive index change through theelectro-optic effect, providing the desired switched waveguidingsection. As described elsewhere, this waveguide segment is alsoconfigured and aligned to optimize the coupling of the input mode 851into the output mode 854. As an alternative in this and any of the TIRswitch implementations, the output waveguide may originate at the inputwaveguide with negligible gap. This alternative has higher insertionloss in the switch off (straight through) configuration, but it has asimpler structure.

Referring to FIG. 33, there is shown a two position waveguide routerusing a poled segment, which is not based on total internal reflection.The poled region 866 forms an electrically excitable waveguide segmentwhich crosses the input waveguide 862 at a small angle. When the fieldis applied, the index in the segment 866 is increased, while the indexin the adjacent region in the input waveguide is decreased. Thus, theinput beam 880 is at least partially coupled into the poled waveguidesegment. When the switch is off, the input beam continues to propagatethrough the input guide to form an unswitched output beam 882. The smallangle may be tapered adiabatically, forming a low loss waveguide bend,if it is desired to switch all or most of the input light into theoutput guide 864 to exit the device as the switched output beam 884.

At least two electrodes are used to apply an electric field across thepoled region to activate the waveguide. A first electrode 870 ispositioned above the poled waveguide segment, while a second electrode872 is positioned adjacent to the first electrode. The second electrode872 is adjacent to the first electrode and may be placed on both sidesof the poled waveguide segment, in order to achieve a high powersplitting ratio. As before, the electrodes are excited by the powersupply 846 through conductors 844, and a planar waveguide 835, or theelectric field falloff with depth, or one of the other approachesdescribed herein is used to obtain vertical confinement for the switchedpropagating modes.

Referring to FIG. 34, several poled TIR switches are placed side by sideto form an array 900. The poled regions 912 and 914 forming the TIRinterfaces are placed one after the other along the waveguide 910. Eachpoled region has the same crystal orientation, with the z axis of thecrystal in the regions 912 and 914 reversed relative to that of theremainder of the crystal. The other aspects and many variations of thisconfiguration have been described above in reference to FIG. 30.

Each of the switches are individually activated using a multi-outputvoltage control source 926, which is connected to the electrodes withwires 928. When all switches are off, the input beam 902 propagates downthe input waveguide 910 to form an unswitched output beam 904 withnegligible loss. If the first switch is on, then the input beam reflectsoff the first TIR interface to form a first reflected output beam 908 inwaveguide 916. If the first switch is off and the second on, the inputbeam reflects off the second TIR interface to form a second reflectedoutput beam 906 in waveguide 918, and so on for the subsequent switches.This multiple switch structure can be extended to n switches.

An electrode is laid out over each TIR interface as described above. Oneor more of the electrodes 920, 922, and 924 serve as the cathode for oneswitch and the anode for another. For example, a voltage is appliedbetween the second electrode 922 and the first and third electrodes 920and 924 to activate the second switch forming the output beam 906. Anelectrode 922 that acts as both an anode and a cathode should preferablyextend adjacent to the TIR interface of the prior poled segment 912while also covering the TIR interface of one poled region 914 and onewaveguide segment of one poled region 914. Only a portion of thestructure is shown, with two complete poled segments 912 and 914 and onecomplete electrode 922. This structure can be replicated into n switchesby aligning duplicate complete electrodes and poled segments.

In order to avoid crosstalk in the channels, the voltage on theelectrodes may be applied in such a manner that the input beam does notsee any electro-optic index changes until it enters the region of theactivated switch. For example, to activate the TIR interface of thesecond poled region 914, a voltage may be applied between electrodes 922and 924, keeping the same potential on electrodes 920 and 922 and priorelectrodes.

Although the total length of the poled regions is longer than L, thedistance occupied along the waveguide by a given region is equal to L bydefinition. A linear array of TIR switches with a 100% packing densitywould therefore have new poled regions starting every distance L. Thisis called 100% packing density because at this density the adjacentregions just barely touch each other at the inside corner of the poledregion in the waveguide. Having adjacent regions touch each other isdisadvantageous because some of the light guided in the previous poledstructure can leak out into the next poled structure where thestructures touch.

We have noted above that the corner which touches the preceding poledregion is formed by two vertical faces of the poled region whoseplacement is not critical. By moving these faces so that the width ofthe poled region is thinned on the side of this inside corner, it can bearranged that the regions no longer touch each other, reducing the leakof optical energy. For example, the inside corner can be moved to themiddle of the waveguide by halving the length of the face whichtraverses the waveguide at 90°. The face which used to parallel thewaveguide now parallels the TIR interface, and becomes a criticallypositioned surface. We call the poled regions with this geometry "densepacked" poled regions. (There are other ways the objective of minimizingthe light leak may be accomplished, such as adding a seventh verticalface between the two noncritical faces, but the alternative justdescribed has another advantage in dense packing.)

FIG. 35 shows a configuration wherein the linear density of switches isbe doubled by using the dense packed geometry for the poled region andreversing the polarity of the adjacent poled regions. The interfaces ofthe poled regions transverse to the waveguide are now identical but fora translation along the axis of the waveguide. The poled regions willtherefore stack solidly along the waveguide, doubling the switchdensity. In fact, only the reverse poled region is fully spatiallydefined, since the other region has the same poling direction as thesubstrate (in the optimal case where the substrate is fully poled). Tworegions 952 and 954 of reverse poling are shown in FIG. 35. The TIRinterfaces can be thought of as the first face or the input face and thesecond face or the output face of the poled region where unswitchedlight travelling in the waveguide 950 potentially enters or leaves,respectively, the unexcited poled region.

The TIR interface for the output beam 946 is formed between the poledsubstrate and the first (input) face of the reverse poled region 952,and is excited by electrode 966. The TIR interface for the output beam947 is formed between the second (output) face of the reverse poledregion 952 and the poled substrate, and is excited by electrode 967. TheTIR interface for the output beam 948 is formed between the poledsubstrate and the first face of the reverse poled region 954, and isexcited by electrode 968. The TIR interface for the output beam 949 isformed between the second face of the reverse poled region 954 and thepoled substrate, and is excited by electrode 969. The electrodes extendabove the respective TIR interfaces, and along the switched waveguidesegments which connect to the permanent output waveguides 956, 957, 958,and 959. Preferably, one or more of the electrodes 966, 967, 968, 969and 970 serve as the cathode for one switch and the anode for another.Each electrode therefore extends parallel to and alone the full lengthof the TIR interface of the previous switch.

Each of the switches is individually switchable by applying electricfields with voltage source 926 via conductors 928. When all switches areoff, the input beam 942 propagates down the bus waveguide 950 to form anunswitched output beam 944. When the first switch is on, the input beamreflects off its respective TIR interface to couple into the firstoutput waveguide segment 956 to form a first reflected output beam 946.For the subsequent switches, the input beam reflects off the respectivesubsequent TIR interface to couple into a waveguide segment 957, 958, or959 to form a reflected output beam 947, 948, or 949. The voltage on theelectrodes is typically set so that there is no optical interferencefrom adjacent switches: all preceding switches are off. This can beaccomplished for example by maintaining all the preceding electrodes atthe same potential as the switched electrode. This multiple switchstructure can be extended to n switches.

It is desirable to extend the upstream end of the dense packed poledregions significantly beyond the edge of the input waveguide 950,maintaining the angle of the vertical surfaces with respect to thewaveguide. This extension captures the full exponential tail of theinput waveguide mode, and pushes the remaining noncritically positionedsurface of the extended dense packed poled region out of the waveguide950, thereby diminishing the optical loss. (Upstream and downstream aredefined in relation to the direction of propagation of the input beam942.)

If the switched waveguide segment of the poled regions is designed asdescribed above in reference to FIG. 30, the separation of the outputwaveguides becomes equal to their width in the highest density packing,so that they merge into a planar waveguide. While a planar outputwaveguide may be useful for some applications, the output waveguides maybe separated using a second poled TIR interface within each switch. Theuse of two TIR interfaces in a switch has been described in reference toFIG. 31. Note that in the case of FIG. 35, the geometry of the poledregion is slightly different to accomplish the stacking. The "outputwaveguide" section of the extended dense packed poled regions is rotatedabout the end of the first TIR interface to an angle 3θ relative to theinput waveguide 942, maintaining the parallelism of its faces. This"output waveguide" section therefore becomes a second TIR reflectorsegment.

The width of the second TIR reflector segment is about 50% larger thanthe input waveguide. The mode propagating in the second TIR reflectorsegment is unconfined on its inner side for a distance of about 2W/sinθwhere W has been defined as the waveguide width. Any diffraction whichoccurs on this side will result in reduced power coupling into theoutput waveguides 956-959. It is desirable to keep this distance lessthan about a Rayleigh range. In the case of a 4 μm wide waveguideoperating at a TIR angle of 4.5°, the total unconfined distance is about100 μm, which is approximately equal to the Rayleigh range for a bluebeam. One solution to optimizing the performance of an array of suchswitches lies in adding a permanent reduction of index of refraction(without degrading the electro-optic coefficient) in a strategiclocation within the second TIR reflector segment. This strategiclocation is the zone bounded by the inside wall of the extended densepacked poled region, and by the inside wall of the poled region 836 asdefined in reference to FIG. 31. The permanent index of refractionreduction defines a permanent waveguide boundary at the optimal locationfor confinement of the mode as it is reflecting from the two successiveTIR mirrors. The added index reduction tapers to zero as it approachesthe input waveguide, and the loss added to the input waveguide can bereduced sufficiently by truncating the index reduction regionsufficiently far from the guide. The index reduction also does notinterfere with the TIR function of the previous TIR interface (indeed,it helps).

Thus, the switched beam reflects from two consecutive TIR interfaces,doubling the total deflection angle of the switch to 4θ. By doubling theoutput angle, space is now made available for output waveguides of widthequal to the input waveguide, with a separation equal to their width inthe densest configuration.

The output waveguides connect to the poled region in FIG. 35 at thefinal corner of the second TIR reflector, at an angle θ relative to thesecond TIR interface and optimally aligned to collect the lightreflecting off the second TIR interface. Preferably, the two TIRreflectors for a given switch are connected without an interveningwaveguiding segment. This minimizes the path length that the deflectedbeam must travel in the poled waveguide, which may have a higher lossthan a permanent channel waveguide due to wall roughness and asymmetry.

In an alternate poling boundary structure, the boundary between twoadjacent poled regions may be a curved TIR structure. The mode of such astructure is again a whispering gallery mode, possibly modified by someconfinement on the inside boundary of the waveguide. The radius ofcurvature of the poling boundary is made small enough so the whisperinggallery mode matches well with the waveguide mode for large powercoupling between the two types of guide, yet large enough for practicaltotal internal reflection to occur for the distribution of angles withinthe mode.

FIG. 36 shows a dual crossing waveguide structure 980 for higher packingdensity. This structure incorporates two innovations: an asymmetric losswaveguide cross 997, and 90° mirrors 976 and 977. The density isincreased with the addition of a second input waveguide 982 parallel tothe first input waveguide 984, on the same surface of the substrate 981,effectively doubling the packing density. The switching elements 983 and985 have been illustrated schematically as one of the variants of thepoled TIR switch described above, but can alternatively be anyintegrated optic switch described in the literature, so we do notdescribe the switch in detail here or in the FIG. 36. (The switches mayalso be implemented in alternate ways described herein such as thegrating switches described in reference to FIG. 7, the coupler describedin reference to FIG. 10, the splitter described in reference to FIG. 25,and the guiding switch described in reference to FIG. 33.)

A first input beam 992 propagates down the first waveguide, while asecond input beam 994 propagates down the second waveguide. The twobeams may originate from distinct sources or from the same opticalsource via an active or passive splitter. When the corresponding switchis off, the input beam 992 and 994 propagate through to form theundeflected output beam 993 and 995, respectively. If the correspondingswitch is on, the first input beam 994 is deflected into the output beam996, while the second input beam 992 is deflected into the output beam998.

In the asymmetric waveguide cross 997, two waveguides cross each otherwith the index of refraction profiles adjusted to minimize the loss inone guide at the expense of somewhat higher loss in the other. Thecrossing guides are laid out at a large angle with respect to each other(herein illustrated at 90°), in order to minimize the crossing loss.Referring to the geometry of FIG. 36, the second deflected beam 998crosses over the first waveguide 984 (in this case so that the switchedoutput light beams can propagate in parallel output waveguides 986 and988). The waveguide 988 is broken at the crossing point with thewaveguide 984, leaving the gaps 990 and 991. This is done to minimizethe loss in the waveguide 984, producing an asymmetric loss structurewith higher loss in waveguide 988 than in waveguide 984 in the crossingregion. For later convenience, we say that the asymmetric cross "points"along the waveguide with lower loss. The asymmetric cross 997 pointsalong the waveguide 984. If the gaps 990 and 991 are wider than severalexponential decay lengths for the mode in the guide 984, the crossstructure will provide essentially no additional loss to the waveguide984. A large number of asymmetric cross structures may then be sequencedpointing along the waveguide 984 to produce a low loss waveguidecrossing many others. The gaps 990 and 991 will produce some reflectionand scatter to the beam 998 propagating in the broken waveguide 988, andthe width of the gap may be minimized subject to the combinedconstraints of desired low loss in the two waveguides. To minimize theoptical loss from the beam 998 propagating in the waveguide 988 at thecross structure, the index profile transverse to the axis of propagationof the guide may be modulated or tapered along the axis of the guide.The goal is to maintain very low loss in the waveguide 984 whileminimizing the loss in 988. This purpose is achieved if the index ofrefraction change in the regions adjacent to the guide 984 is small andslowly varying compared to the index of refraction change of thewaveguide 984 itself. (All index of refraction changes referred to arerelative to the substrate.)

The loss in the second waveguide has two components: one due toreflection from the index discontinuities, and one due to diffractivespreading. The reflection loss is determined by the magnitude of theindex change in the waveguides, and by its taper profile at the ends andsides of the waveguides. For example, if the index change at the core ofthe waveguides is the same in both at Δn=0.003, the net reflection lossat the four interfaces will be less than 5%, neglecting corrections dueto the exact index profiles which can reduce the reflection. Thediffractive loss is even lower because the gap width is typically muchless that the free space Rayleigh range. If, for example, the narrowestmode dimension is the depth, at 2 μm, then the Rayleigh range is 55 μm,assuming an index in the material of 2.2 and a wavelength of 0.5 μm. Thediffractive loss at each gap is less than 1%, assuming a 3 μm wide gap.If the waveguide depth is 4 μm, the diffractive loss is substantiallysmaller. The diffractive loss may be minimized by increasing thedimensions of the waveguide relative to the gap size.

In general, the "gaps" 990 have an index of refraction distributionadjacent the crossing region. This index of refraction distribution isdefined relative to the index of refraction of the substrate. The indexof refraction in the gaps may taper from a value equal to the index ofrefraction distribution of the waveguide 988 to another value adjacentthe crossing region. The important part of the crossing region is thevolume within which propagates the optical mode of the waveguide 984. Tominimize the loss in the waveguide 984, the index of refraction adjacentthe crossing region in this important part is much smaller than theindex of refraction distribution within the waveguide 984.

The crossed waveguide geometry with asymmetric optical loss may becombined in many geometric variations. For example, three or more inputwaveguides may be used with multiple crossing points where switchedoutput waveguides traverse input waveguides. The selection of preferredwaveguides, preferred in the sense of having its loss minimized at thecrossing point, can be also done in many ways. We have discussed anexample in which the preferred guides are parallel. However, in a morecomplex system, there may be preferred guides which cross each other aswell as crossing unpreferred guides. The selection of how to accomplishthe crossings of the preferred guides depends on the application. Thewaveguide crossing structures in a device may be any combination ofasymmetric loss crossings and symmetric loss crossings where the gapwidths are zero.

For switches that deflect the beam at a small angle (such as a TIRswitch), additional beam turning means such as 976 and 977 may beprovided, in order to achieve the desired large angle of intersection atthe waveguide cross. The beam turning means 976 and 977 is preferably avertical micromirror, and is installed at a fixed position. Eachmicromirror is formed by removing the substrate material within itsvolume, leaving a flat vertical surface (preferably with low roughness)adjacent to the waveguide and oriented at such an angle so as to directthe reflected light optimally down the output waveguide 986 or 988. Themicromirrors may be fabricated using conventional processing techniques,including laser ablation with, for example, a high power excimer laseror ion beam etching, both of which might define the mirror geometry withthe aid of a mask. The volume may be filled with a low index, low lossmaterial such as aluminum oxide or silica to prevent contamination ofthe mirror surface, and to maintain the total internal reflectionproperty of the mirror.

The angle of the micromirror relative to an input of one of thewaveguides is preferably adjusted to provide total internal reflection.The thickness of the micromirror volume in the direction normal to itsreflective surface is preferably much greater than a wavelength of lightin order to minimize leakage through the micromirror volume of theevanescent tail of the reflected light wave. The angle relative to theother waveguide is adjusted so that the mean propagation direction ofthe reflected beam is parallel to the central axis of the otherwaveguide. The location of the micromirror is adjusted to optimize thecoupling of the light from one waveguide to the other. The location ofthe mirror in the junction region is preferably adjusted so that the"centers of gravity" of the two beam profiles illuminating the mirrorsurface are at the same place. The length of the mirror transverse ofthe incident and reflected beams is greater than about twice the widthof the waveguide to reflect essentially the entire mode, including theexponentially decreasing intensity in the beam tails. Light input fromone of the waveguide modes diffracts through the waveguide junctionregion to the micromirror, reflects, and diffracts back through thewaveguide junction region at the reflected angle before coupling intothe output waveguide mode. The junction region between the twowaveguides in the vicinity of the mirror is optimally kept smallcompared to the Rayleigh range of the unconfined beam, which can beaccomplished with waveguides having widths in the 2 to 5 micron range.

The structure of FIG. 36 makes possible a large interdigitated array ofswitched light distribution waveguides. The entire structure 980 may bereplicated many times along a pair of input waveguides, producing a setof interleaved output waveguides with a simple pattern of alternatingparentage (in this context, parentage means deriving optical power froma specific "parent" input waveguide). Each input waveguide may beconnected to a large number of output waveguides as long as theswitching elements have a very low insertion loss, as is the case forthe elements listed above and described herein. Because of theasymmetric cross structures, adding more input waveguides above theothers (with additional switches, micromirrors, asymmetric waveguidecrosses, and interleaved output waveguides) does not significantlyincrease the loss of the lower input waveguides or affect their abilityto distribute light over a long distance to many output waveguides. Itwill increase moderately the optical source power required for eachadditional input waveguide in order to deliver the same power to the endof their respective output waveguides. As many input waveguides asdesired may be used in parallel to distribute a potentially large totalpower of light. Their output waveguides may be interleaved in manyalternative patterns using the approach of FIG. 36. The same result maybe achieved using grating reflectors in the place of the TIR switches.If the grating reflectors are oriented at a large angle to the inputwaveguides, the micromirrors are also no longer needed.

The structure described in the previous paragraph is a one-to-manyarchitecture in that it has one output per switch with a multiplicity ofswitches per input. There is no way to connect many inputs into the sameoutput. What is needed is a many-to-one architecture. The many-to-manyconfiguration is then obtained by combining the one-to-many and themany-to-one configurations.

FIG. 37 shows an array 1060 of waveguides with TIR switches arranged ina many-to-one configuration. In the structure shown, two Inputwaveguides 1072 and 1074 switch two input beams 1062 and 1064 into anoutput beam 1070 in one output waveguide 1076. The input TIR switches1090 and 1092, and the output switches 1094 and 1096 have been describedbefore in reference to FIGS. 30-32 and 36, so they are shown onlyschematically, leaving off many elements (such as the electrodes, thecontacts, the power supply, the controller, the vertical confinementmeans, the depth of the poled regions, the type of output waveguideconfinement) for clarity. The input TIR switch is arranged with the beampropagating in a forward sense as described in reference to FIG. 36,while the output TIR switch is arranged with the beam propagating in areverse sense. The switches 1090 and 1092 are switched at substantiallythe same time, as are switches 1094 and 1096, because both are requiredto accomplish injection of power into the output waveguide 1076. Asdescribed in reference to FIG. 36, when the switches 1090 or 1094 areon, a fraction of the beams 1062 and 1064 are switched, respectively,into the waveguide 1078 or 1084. The remainder of the input beamspropagates along the extension of the input waveguide into an outputpath as beams 1066 or 1068, which may be used in some other component orbrought into a beam dump for absorption or scatter out of the system.Micromirrors 1082 and 1088 are provided to reflect the beams fromwaveguides 1078 and 1084 into the waveguides 1080 or 1086, respectively.In their on condition, TIR switches 1092 or 1096 receive the beamspropagating in the waveguides 1080 or 1086, respectively, forming theoutput beam 1070. If it is desired to switch the beam 1062 into theoutput beam 1070, clearly the switch 1096 and all subsequent switchesmust be off. (It would otherwise reflect much of the desired beam out ofthe waveguide 1076.) A similar constraint applies for all the otherswitched beams in multiple switch arrays.

The substrate 1098 is processed as described herein to produce thestructures illustrated. When the switches 1090 or 1094 are off, theinput beam propagates through the switching regions 1090 or 1094 withnegligible loss, traverse the waveguide 1076 (in an asymmetric cross ifdesired), and emerge as output beams 1066 or 1068, respectively,possibly for use as inputs to additional switches.

Additional input waveguides may also be provided, coupling into thewaveguide 1076 (or not coupled, as desired), in a modified repetition ofthis structure in the direction of the output beam 1070. Additionaloutput waveguides may also be provided, coupled if desired to the inputwaveguides 1072 and/or 1074, in a modified repetition of this structurein the direction of the beams 1066 and 1068.

FIG. 38 shows an array 1210 of grating reflectors in a many-to-manyconfiguration. In the structure shown, two input waveguides 1222 and1224 switch two input beams 1212 and 1214 into two output beams 1220 and1221 in two output waveguides 1226 and 1228 which abut or encounter theinput waveguides. The grating switches 1230, 1232, 1234, and 1236containing the gratings 1238, 1240, 1244, and 1246 have been describedbefore in reference to FIGS. 7, 8, 12, and 13, so they are shown onlyschematically, leaving off many elements (such as the electrodes, thecontacts, the power supply, the controller, the vertical confinementmeans, the depth of the poled regions, the tapering of the poled regionsor electrode spacing) for clarity. When the switches 1230 or 1232 areon, a fraction of the beam 1212 is switched into the output beams 1220or 1221, respectively. The remainder of the input beam propagates alonga continuation of the input waveguide into an output path as beam 1250,which may be used in some other component or brought into a beam dumpfor absorption or scatter out of the system. When the switches 1234 or1236 are on, a fraction of the beam 1214 is switched into the outputbeams 1220 or 1221, respectively. The remainder of the input beampropagates along a continuation of the input waveguide into an outputpath as beam 1252, which may be used in some other component or broughtinto a beam dump for absorption or scatter out of the system. It shouldbe understood that the structures admit to bidirectional propagation.

The substrate 1248 is processed as described herein to produce thestructures illustrated. When the switches are off, the input beamspropagate through the switching regions (in which the waveguides may beconfigured as an asymmetric cross if desired), and emerge as outputbeams 1250 and 1252, respectively, possibly for use as inputs toadditional switches. The waveguides may cross each other in simplelarge-angle junctions as shown, or the junctions may be asymmetriccrosses, which do not substantially affect the placement of the gratings1238, 1240, 1244, and 1246. Note that the gratings may in fact be partsof a single large grating which covers the substrate and which is onlyactivated in the regions of the different switches by the desiredelectrodes. If the gratings are constructed from poled domains, forexample, this allows the entire substrate to be poled for the gratings,which may be simpler in production. Alternatively, the gratings could bearranged in stripes or other groupings.

Additional input waveguides may also be provided, coupling into thewaveguides 1226 or 1228 (or not coupled, as desired), in a modifiedrepetition of this structure in the direction of the output beams 1220and 1221. Additional output waveguides may also be provided, coupled ifdesired to the input waveguides 1222 and/or 1224, in a modifiedrepetition of this structure in the direction of the beams 1250 and1252.

FIG. 39A shows schematically an example application of the alternativeswitch arrays in the n×n communications routing application. In thisapplication, the optical power in n input optical channels is to berouted to n output optical channels with minimal loss and minimalcrosstalk. A controller sets up an addressable path between one channeland another. A simple square array is formed by repeating the structureof FIG. 38 until n inputs are arrayed on the left and n outputs arearrayed on the bottom, with switches at all n² of the waveguideintersections. The intersection angle may be any convenient angle. Inthis structure, the switching of any channel into any other isaccomplished by activating one of the switches. The light beams crosseach other at the waveguide crosses with a small amount of crosstalkwhich can be reduced by optimizing the waveguide geometry. Thisstructure is capable of independent one-to-one connections between anyinput and any output. Note also that the connections are bidirectionalso that a communications channel can be used equally well, and in factsimultaneously, in both directions. The switches are shown asimplemented with gratings for specificity, but they may be implementedwith dual TIR switches as described in reference to FIG. 37 byreplicating the structure of FIG. 37 forming the n×n inputs and outputs,or with any other optical switching technique now known or yet to bediscovered. Note that in the case of the TIR switches, the optical datapath does not pass through the vertex of the intersection between theinput and the output waveguide. Instead, it passes through anotherwaveguide near the intersection. According to the specific geometry ofthe switch, the input and output waveguides may intersect at a largeangle as shown in FIGS. 37, 38 and 39, or at an oblique angle. The fixedreflectors 1088 and 1082 in the dual-TIR switching geometry may not berequired in the case of the obliquely intersecting waveguides.

In this simple square geometry with n parallel input waveguides, therewill be one input waveguide which can be connected into the closestoutput waveguide with a single switch, forming a best case connectionwith lowest loss. At the other extreme, there will be one waveguidewhich must traverse 2(n-1) waveguide crosses to be switched into thefarthest output waveguide. This worst case connection will have muchhigher loss then the best case connection. To reduce the maximuminsertion loss of the switch array structure, asymmetric cross junctionsmay be used as described in reference to FIG. 36. The loss of the worstcase connection will be best helped with every waveguide cross ittraverses being an asymmetric cross pointing in the direction ofpropagation of the light along either the input or the output waveguide.This structure is clearly not generalizable to the inner waveguidesbecause use of asymmetric switches in the intermediate junctions willhelp some switching paths at the expense of others. What is needed is analgorithm for selecting the optimal direction for the asymmetriccrosses. A good way to dispose the asymmetric crosses is for roughlyhalf of the crosses to point in each direction. Observe that the n(n-1)crosses on the upper left of the diagonal (but not including thediagonal) are predominantly used to distribute energy to the right.These crosses therefore should point along the direction of the inputwaveguides, while the crosses on the lower right should point in thedirection of the output waveguides. In a bidirectional structure, thecrosses on the diagonal should be simple symmetric crosses, hereincalled the simple diagonal arrangement of the asymmetric crosses. Otherarrangements may be used according to different usage patterns, but thisis a good general purpose arrangement.

A n×m (where n>m) arrangement will permit full connectivity only betweenm "input" lines and m "output" lines. Here, "input" and "output" areonly used for identification purposes since all lines are bidirectional.The additional n-m "system" lines may be useful for system control inboth monitoring and broadcast functions. If line A wishes connection toline B, for example, it would send system requests for that functionuntil answered. Line m+3, for example, might be dedicated to scanningall the "input" lines for system requests. (To provide a similar line tomonitor the "output" lines, a larger matrix of lines is required, suchas the n×n matrix shown in FIG. 39A where m lines are dedicated to usersin a sub group of m×m lines. A line such as line n-2 may then be used tomonitor the "output" lines.) In monitoring, the system will turn onsuccessive gratings corresponding to the "input" or "output" lines, anddetect whether the line is active. Some power will be switched into themonitor detector by the successively switched-on gratings in line withthe monitor detector if any one of the monitored lines is active. Anactive line will have an activated reflector connecting it to anotherselected line. However, the activated reflector will leak some powerthrough to form a beam which can be detected by the monitor detector.When the monitor detector connected to line m+3 in this example switcheson the switch 1255 (drawn as a grating switch for specificity) andreceives the request from line A, the control system will have to checkwhether line B is busy. When the connection is made to line n-2 throughswitch 1253, the residual beam which leaks past the line B connectionswitch will alert the system that line B is active. If no activity issensed, a system request can be sent to both lines A and B (possiblythrough the same monitor line if it has multiplexed send/receivecapability, or possibly through a separate system line), and the switch1254 can be closed to establish the connection.

The broadcast function is not feasible from lines within the basic m×mswitching block which is used for one-to-one connections, because evenpartially turning on the required row of switches corresponding to allthe outputs from a given input would interfere with some of the alreadyestablished and potentially active communications connections betweenother channels. Broadcast is best accomplished from the system lineswhich are "outside" of the m×m switching block illustrated in FIG. 39A.(The "inside corner" of the geometry is the best case waveguideconnection with the lowest loss, between lines 1 on the "input" side andline 1 on the "output" side.) Line C is shown to be actively connectedto most or all of the "output" lines in FIG. 39A by means of gratings1256 as an example of broadcast. The switches 1256 on line C must beonly partially turned on so that sufficient power is delivered to each"output" line. A similar protocol may be used to prevent collisionsbetween channels in the case of broadcast as in the case of simplecommunications connection. Broadcast connections would only be set upwith inactive channels, and the system can group channels togetherand/or wait for individual channels to permit broadcast to them.

To increase switching efficiency, the waveguides may be large multimodewaveguides, which in the case of a single mode communications networkwill be connected to the single mode input and output ports 1 through mwith adiabatic expanders described elsewhere herein.

The entire structure described above in reference to FIG. 39A is usefulas an asynchronous transfer mode (ATM) switch, or in any point-to-pointswitched communications application. One useful variation of thestructure is for multiple wavelength operation in a WDM network.Wavelength selective optical switches can be implemented as describedherein by using poled grating switches, or by using tunable fixedgratings which tune into and out of a specified communications band. Ina WDM network, the desire is to switch a specific wavelength betweenchannels without affecting other wavelengths which may be travelling(bidirectionally) in the same channel. With a tunable switch which canselect a frequency of reflection while essentially transmitting theother set of frequencies in the WDM spectrum, the simple geometry ofFIG. 39A is appropriate. However, if a switched grating is used whichhas a single frequency of operation, separate connection paths arenecessary for each wavelength.

FIG. 39B shows a switched WDM communications network 1260 with separatepaths for each frequency used in the network. This example is for a twofrequency WDM network, but may be generalized to any number offrequencies of communication. Three "input" waveguides 1276 are shown inFIG. 39B connected to three ports 1a, 2a, and 3a, and three "outputwaveguides 1276 are shown connected to three ports 1b, 2b, and 3b. Thewaveguides form nine intersections. At each intersection, there arethree additional optical paths connecting each "input" and each"output". The additional paths are identical in this example, andconsist of three types. The first type 1266 of optical path consists ofa pair of fixed frequency switched reflectors both capable of reflectingthe first one of the two signal frequency bands of the WDM system. Thereflectors are preferably gratings transverse of the "input" and the"output" waveguides associated with the intersection, and reflect powerin the first frequency band between the corresponding waveguide and anadditional waveguide segment connecting the two gratings. The secondtype 1268 of optical path consists of a second pair of fixed frequencyswitched reflectors both capable of reflecting the second one of the twosignal frequency bands of the WDM system. Again, the reflectors arepreferably gratings placed transverse of the respective waveguides andreflect power in the second frequency band between the correspondingwaveguides and an additional waveguide segment connecting the second twogratings. The third type 1270 of optical path consists of a pair offrequency independent switched reflectors both capable of reflectingboth signal frequency bands of the WDM system. This third type ofoptical path may be implemented as the pair of TIR reflectors connectedby waveguides and fixed mirror (described in reference to FIG. 37).

In this case, ports 1a, 2a, 1b, and 2b plus the associated waveguides1276, 1277 form a 2×2 switching network capable of switching twofrequency channels simultaneously between any "input" port and any"output" port. System control ports 3a and 3b with associated waveguides1276, 1277 provide monitoring and system communication functions. If thefirst frequency of the WDM system is desired to be switched between port2a and 1b, for example, the two switches associated with the opticalpath of type 1266 at the intersection of the waveguides connecting toports 2a and 1b are turned on, routing optical power at the firstfrequency between ports 2a and 1b through the waveguide connecting thetwo switches. If all frequencies associated with a given port are to berouted into another port, the switches and optical path of type 1270 areturned on at the intersection corresponding to the two ports. Theoptical paths 1270 are really superfluous in a 2×2 network because toswitch both WDM frequencies between any two channels, both correspondingpaths 1266 and 1268 may be activated. However, in a high ordercommunications network with many WDM frequencies, a single all-frequencyconnection is desirable since it will have the lowest loss.

FIG. 40 shows a two dimensional one-to-many routing structure. A firstrow of waveguide routing switches connects optical power from an inputwaveguide into columns of pixel waveguides. Again, no details of theswitches are shown; they are shown schematically only as gratings, butmay be implemented in several different ways. A two dimensional array of"pixel" switches routes power out of the pixel waveguides at "pixellocations". (What happens to this power at the pixel locations dependson the application.) Two levels of switching are used to reach all thepixels. This structure may be used for display, to actuate or controlprocesses or devices, or to read certain types of data. In the lattercase the direction of the power flow is reversed, and the deviceoperates as a many-to-one routing structure.

An input optical beam 1342 propagates in an input waveguide 1352 and iscoupled into one of many pixel waveguides 1354 by one of a twodimensional array 1356 of switching elements. The switching elements1364 may be implemented as grating switches as described above inreference to FIGS. 7, 8, 12, 13, and 38, or they may be TIR switches asdescribed in reference to FIGS. 30-32 and 37, or they may be any otherswitchable element. The beam 1344 is shown being switched by switchelement 1358 into a pixel waveguide whereupon it is switched for asecond time by switch element 1360, forming beam 1346 which propagatesinto the pixel element 1362. The pixel elements 1366 may be separatedfrom the waveguides 1354 by waveguide segments as shown, or they mayabut the waveguides at a short distance so that little of the switchedlight passes by the pixel elements.

In the case of the display application, the pixel elements may be forproducing emission of the light 1346 out of the plane of the substrate1348. The pixel elements may then be roughened patches on the surface ofthe substrate 1348, or angled micromirrors, or roughened angledmicromirrors for light diffusion, or phosphor-filled pits, or othermeans of producing visible light. In the case of the display, the inputbeam 1342 may contain several colors, in which case the waveguides arecapable of guiding all of the colors and the switches are capable ofcoupling all of the colors. The waveguide switches are scanned in asequence to produce the image of the display. A grating switch isimplemented as a multiple period grating, but the TIR switch needslittle modification for this purpose. The waveguides, if single mode,must effectively guide the shortest wavelength beam. The input beam 1342is preferably modulated externally (including all its color components)so that the switching elements are simple on-off devices. Note that asingle row electrode may be disposed across the columns of waveguides toactuate a row of pixel switches if the pixel elements are arranged in amore-or-less straight line and are connectable electrically along a row.

In the case of a projection display, a additional lens structure isrequired to collect the light emitted by all the pixels in the array andrefocus them on a screen at a (large) distance from the lens. The lensshould preferably have a good off axis performance so that the focalplane is reasonably flat at the screen, and it should have a largeenough numerical aperture (NA) to collect most of the light emitted bythe pixel array. It would be advantageous to couple a lens array to thepixel structure to reduce the divergence of the beam produced by theindividual lenses, reducing the (costly) NA requirement on theprojection lens. Another way to achieve this is again to taper thewaveguides to the largest possible size at the pixel. It is relativelyeasy to taper the pixels to a large transverse size, but difficult toobtain a very deep waveguide. Large pixels may be made by coupling awide waveguide with a long grating coupler.

The light distributed in the routing structure may also be used toactivate processes, as for example in the case of a DNA reader or anallergy reader, or a protein reader. In each of these specific cases, aseparate array of DNA or allergens or proteins is prepared withfluorescent tags which can be excited by the light. One type of moleculeor one preparation of molecules may be arranged for excitation over eachpixel. The light is scanned electronically among the different pixels,and the speed and order of the scanning may be determined according tothe results. The fluorescence may be collected for detection by anexternal lens and detector. However, for some applications, it isadvantageous for the pixel (and its lens) and waveguide structure itselfto collect and guide the emitted radiation to an optical energydetection means as well as to control the emission of the source light.Depending on the desired light illumination and collection geometry, thelens may be a collimating lens, a refocussing lens, or even, conceivablya lens to produce a diverging beam. A collimating lens is separated fromthe end of the waveguide by the focal length of the lens so that thetransmitted (and collected) beam is essentially parallel. Collimatinglenses are most useful if there is a large volume of material to betraversed by the interrogating light beam. A refocussing lens isseparated from the end of the waveguide by the object distance, theinverse of which is related to the difference between the inverse of theimage distance and the inverse of the focal length, where the imagedistance is the distance from the lens to the desired image beam spot.The refocussing lens is used if it is desired to concentrate the sampleinto a small spot and to illuminate and/or read it from a waveguide. Adiverging beam is created by a lens separated by less than its focallength from the end of the waveguide. The output beam from a simple lensis not necessarily round if the divergences of the wave approaching thelens are different in the two planes. The simplest way to make a beamround (for minimum spot area after refocussing) is to start with a roundbeam at the end of the waveguide, which may be accomplished by design inthe waveguide, or by tapering the waveguide. The lens preferably has theappropriate numerical aperture to admit the entire wave from thewaveguide and focus it to a diffraction limited spot or collimated beamaccording to the application.

The pixel element 1362 may be any of the elements mentioned above inthis case, and it may be associated directly with the material to beactivated, or indirectly as by alignment with an external plate to whichthe material has been conjugated. Each pixel element may contain a lensaligned as described above so that a switch array may be coupled with alens array with the image beam spots in a substantially common plane offocus. (Substantially common, in this case, means within a Rayleighrange or so of the true plane of focus, which may be quite distorted dueto aberration. Use of a type of reflector instead of a diffuser in thepixel element 1362 is preferred if the routing structure is also used todetect the fluorescent emission: the reflector couples the emission backinto the waveguide whence it came. This coupling is maintained for aslong as the switches for a given pixel are activated. If desired, thelight source may be switched off prior to switching to another pixelelement in order to resolve the decay of the emission.

Used as a data reader, the sense of the light propagation is reversedfrom that illustrated in FIG. 40. Light from a device containing data iscollected at the pixel elements and coupled into the routing waveguidestructure which guides it back out the input waveguide 1352. Connectedto the waveguide 1352 is a detector to read the data. The detector maybe simultaneously connected to the waveguide via a beamsplitter betweenthe waveguide 1352 and the light source used for illumination of thedata media. The pixel elements 1366 (or simply "pixels") are preferablycoupled with the data spots via lenses to collect the light routedthrough the structure 1350 and direct it to the data medium. The lenscoupling also serves for collecting reflected or otherwise emitted lightfrom the data medium and refocussing it on the end of the waveguidecoupled to the pixel element. The data may be in a target volume, inwhich case the lens may be configured to collimate the light beam 1346.The data may be on a target surface, in which case the different pixelelements may correspond to different tracks on the rotating disk of amagneto-optical data storage surface, for example, or of a CD. The lensis configured to refocus the light from the pixel to the data spot in adiffraction limited way. By associating the different pixels withdifferent tracks, track-to-track switching may be accomplishedelectronically with essentially no delay time.

The different pixels may also be coupled to different planes on the datamedium. This is useful for reading data which have been recorded inmultiple planes on the medium, to increase total storage capacity.Switching between the planes may also be accomplished electronically byswitching among pixels coupled to the different planes.

In addition, several different pixel elements may be focussed tolocations separated by a fraction of the track separation transverse of(preferably normal to) a given track. When the track wanders, positivetracking may be accomplished electronically by switching between pixels,instead of mechanically. A sensor and electronics is needed to detecttrack wander, and a controller for switching to the desired pixels. Thesignal strength or the signal to noise ratio (SNR) may be detected inthe different channels to determine the preferred (best aligned)channel. If the switches along the waveguide 1352 are configured as4-way crosses instead of 3-way, with the fourth leg emerging at the edgeof the substrate, a detector array 1368 may be placed in registrationwith the fourth legs, with individual detectors 1367 individuallyaligned with the columns for detecting the return power from eachcolumn. The optimal reflectivity for the gratings which lie along thewaveguide 1352 is approximately 50% if the detectors 1367 are used, inorder to maximize the return power from the data media on the detectorarray 1368. If a single beamsplitter is disposed in the waveguide 1352upstream of the router structure, its optimal reflection is also 50%.

Note that partial excitation of the different pixels can be achieved bypartial excitation of the switches along either the input waveguide orthe pixel waveguides. The switching elements 1364 can be adjusted bymeans of the applied electric field to vary their reflectioncoefficient. Some of the beam may be transmitted through the desiredpartially-excited switches for use in a second pixel simultaneously.Multiple pixel excitation is of particular interest in the case of trackwander correction, since multiple detectors may also be configured inthe router 1350. For example, if three different pixels on threedifferent columns of the routing structure 1350 are to be simultaneouslyexcited their corresponding pixel column switches will need to bepartially excited. A computation is required of the controller todetermine the appropriate excitation of the multiple switches.Neglecting losses at the switches, to produce equal intensifies on theirrespective detectors for optimal SNR, the first switch corresponding tothe first pixel column should be excited to reflect about 3/16 of theincident light, the second switch corresponding to the second pixelcolumn should be excited to reflect about 1/4 of the remaining lightwhich has passed through the first switch, and the last switchcorresponding to the third pixel column should be excited to reflectabout 1/2 of the remaining light which has passed through the previoustwo switches. About 15% of the incident beam is reflected into eachdetector, assuming 100% reflection from the medium and 100% lightcollection efficiencies. This result is quite good compared with theoptimal 25% of the beam which is received on a single detector in thecase of a single pixel (optimum switch excitation=50% reflectivity).Indeed, more total photons are collected with three beams than with onlyone. Electronic tracking will result in cheaper, faster, and morereliable data read/write devices.

Any combination of these approaches (electronic track switching,electronic data plane switching, and electronic tracking) may be takento increase the performance of a data storage device. A means is alsoneeded to accomplish variable focussing electronically, potentiallyremoving all mechanical motion (except for rotation of the media) fromthe drive. As described below in reference to FIG. 54, electronicallyvariable focussing may be accomplished with a zone-plate lens bychanging the wavelength of the light beam 1342.

As drawn, the routing structure of FIG. 40 is a polarizing structure,with the 90° grating switches reflecting only the TM mode. As a result,only beamsplitting based on intensity can be used. It would be quiteadvantageous to be able to use polarizing beamsplitters because thiswould result in a factor of four increase in the signal strength for agiven light intensity. However, a switching structure capable oftransporting and then separating the two polarizations is required.Although the polarization dependence of the TIR switches may be madenegligible at a sufficiently grazing TIR angle (well below the angle fortotal internal reflection for the TE mode), there is a packing densitypenalty in using very low angle switching geometries.

FIG. 41 shows a linear array of strongly polarization dependent switchesarranged as a data reader 1370. The switches are excited with a beam1342 which is TM polarized and highly reflected in the activated switch1372. Waveguides 1376 and 1378 such as titanium indiffused waveguides inlithium niobate are used which guide both polarizations. The pixelelements are implemented as micromirrors 1374 combined with integratedlenses 1380 and data spots e.g. 1382 arranged in tracks 1384 on a disk1386 rotating about the axis 1388. The orthogonally polarized lightwhich is reflected from birefringent dam spots (or separators) on thedata track is collected by the lens 1380, refocussed back to thewaveguide 1378, and reflected by the micromirror back into the plane ofthe guides with TE polarization. Because the TE mode is both polarizedat Brewster's angle for the grating and has different propagationconstant not phase matched for reflection, it propagates through theswitch without reflection into the detector 1367 of the detector array1368. (Alternately, if the switch is a TIR switch, the reflectivity ismuch less for the TE wave than the TM wave, and a large portion of theTE wave transmits through the switch an impinges on the detector.) Ifanother switch 1373 is actuated instead of the switch 1372, the beamwill propagate to a different pixel 1375 and be focussed according tothe alignment of the pixel 1375 and its microlens 1381 either intoanother data track, or to another data plane, or to the same track butwith a transverse deviation of a fraction of a track width (according towhether the pixel 1375 is for track switching, data plane switching, ortracking control).

Many variations are apparent on the structures described in reference toFIGS. 40 and 41, such as that any of the switches in the router may beoriented differently to change directions of optical propagation in theplane, that multiple types of switches may be used in a single device,and that higher levels of switching may be added. Additional variationsare too numerous to mention.

FIG. 42 shows a switchable integrated spectrum analyzer 930. The inputbeam 921 enters the input waveguide 923 which stops after a certaindistance. The input beam 921 may be propagating in another waveguide orit may be a free space beam which is preferably aligned and mode matchedto optimize the power into the waveguide 923. The device 930 is providedwith a planar waveguide 835 which constrains propagation within theplane. The light beam 927 emerging from the end of the input waveguidediverges in one plane within the planar waveguide until it passesthrough the integrated lens element 925. The integrated lens has anelevated index of refraction relative to the planar waveguide within aboundary defining an optical thickness that reduces approximatelyquadratically from the optical axis. (Or if it has a depressed index,the optical thickness increases approximately quadratically.) The lensmay be fabricated by masked indiffusion or ion exchange, or it may be areverse poled segment excited by electrodes.

The lens 925 collimates the light beam which then passes to at least oneof three grating sections 929, 931, and 933. The gratings are formedfrom individual cells, each cell being a domain, the domains beingdistinguished from the background material and separated by varyingamounts according to the application. The cells have a permanent oradjustable index of refraction difference from the substrate, anddifferent cells may be of different domain types. The permanent domaintypes include, for example, indiffused regions, ion exchanged regions,etched regions, radiation bombarded regions, and in general, regionsformed by any type of index of refraction modifying process. The gratingsections may be fabricated by etching, ion exchange, or indiffusion, inwhich case the gratings are permanent, but they are shown in thepreferred embodiment fabricated from poled domains. Electrodes 932, 934,and 936 are used to individually excite the gratings in combination withthe common electrode 938. The common electrode 938 may be placed on theopposite side of the substrate as shown for simplicity, or surroundingthe electrodes 932, 934, and 936 for low voltage excitation.

The cells in an individual grating may be arranged in alternate ways toform the desired periodicity in the desired direction to supply virtualphotons with the required momenta. They may be arranged in rows todefine certain planes with a virtual photon momentum normal to theplanes with momentum defined by the spacing of the rows. In this case,there will also be virtual photons with momentum along the planes withmomentum defined by the spacing of the cells in the rows. To phasematchretroreflection, the momentum of the virtual photon is exactly twice themomentum of the incident photons, and is directed in the oppositedirection. Any other reflection process has a smaller momentum and isdirected transverse of the incident axis. The period Λ of the rowspacing is therefore fractionally related to the incident wavelength λin that Λ is a fraction of the quantity λ/2n_(eff). In a general case,the cells may be separated by a distribution of distances which varieswith position through the grating so that the virtual photon momentumalong any axis of incidence is determined by the spatial frequencyspectrum (determined through the Fourier transform) of the celldistribution along that axis.

At least one of the gratings 929, 931, or 933 is turned on by adjustingthe potential state of the corresponding electrode. In FIG. 42, grating929 is shown activated. The activated grating contributes virtualphotons to the incident photons, phase matching the scattering processinto an output direction forming a plurality of output beams 935 and 937with different wavelengths, the output beam being separated in angleaccording to their wavelength. The output beams from the activatedgrating 929 pass through the lens 939 which refocusses the output beamsonto a detector array 941. The detector array is a group of sensorsdisposed to receive a portion of the output beams for detection, and arepreferably bonded to an edge of the device 930 as shown. However, if itis desired to integrate the device 930 onto a larger substrate, it maynot be desirable to have an edge of the substrate in this location. Inthis case, other beam extraction methods (such as vertical deflectingmirrors) can be used to deflect a portion of the beams 935 and 937 intothe detector array. The sensing means is placed approximately withinabout one Rayleigh range of the focal plane of the output lens 939. Inthis position, the input beam angles are mapped into output beampositions. Since the gratings map input wavelength into output beamangles, a collimated input beam results in different input wavelengthsbeing mapped into different positions in the focal plane, with spatialresolution of the wavelength spectrum depending on the characteristicsof the grating. The detected power as a function of the location of thedetector in the array 941 is related to the frequency power spectrum ofthe input beam 921. The device 930 is therefore a spectrum analyzer. Itis also a multichannel detector if the input beam is divided intochannels occupying several displaced frequency channels, and the deviceis configured to disperse the channels into predetermined detectors orgroups of detectors.

By switching on different gratings, the device can be reconfigured tofunction in different frequency ranges. For example, if grating 931 or933 is activated, the dispersed light is focussed by lens 939 ontoeither a different detector array 943 or a different portion of anextended detector array 941. The frequency range of the gratings isdetermined by the angle of the grating to the beam, and theperiodicities of the grating. Grating 931 is shown to have a shallowerangle to the beam so that a higher optical frequency range is selectedwhen it is activated. Grating 933 has multiple periodicities transverseto each other so that multiple overlapping frequency ranges can beselected. Multiple frequencies may be mapped into poled regionboundaries as described above in reference to FIG. 18. The poledelements of the grating 933 may be arranged generally in planes orientednormal to the two principle virtual photon momentum directions. Thephasing of the planes is determined by the process for transcribing thecomponent frequencies of the desired grating into domain boundaries.However, the general grating may have momentum components in alldirections, in which case the resulting domain boundaries may notorganize into planes except possibly in a principal direction.

A transmitted beam 913 is refocussed by integrated lens 907 into anoutput waveguide segment 909 to form the output beam 911 which containsat least a portion of the out of band portions of the input beam 921which did not interact with the gratings.

A useful variation of the switched range spectrum analyzer combineselements of FIGS. 42 and 30-35. The basic idea stems from the fact thatthe spectral range of a grating can be shifted by changing its angle, orequivalently the source point. In this variation, a waveguide routingstructure is used to allow the source point to be switched. Waveguideswitches are placed on the input waveguide 923 (and possibly on theemanating waveguides) at one or more locations, producing an array ofparallel source waveguides among which the input light beam 921 isswitchable. The waveguides all end in the same plane, preferably thefocal plane of the input lens 925. The remainder of the spectrumanalyzer remains the same, although with multiple inputs it may not benecessary to have the additional gratings 931 and 933. The separation ofthe multiple switched input waveguides is adjusted according to theapplication to achieve the desired switchable spectral ranges for theanalyzer 930.

FIG. 43 shows a poled acoustic multilayer interferometric structure 953.The incident acoustic wave 972 may be a bulk or a surface acoustic wave.A poled structure is fabricated in the region 955 of a piezoelectricsubstrate 965, containing two types of domains 963 and 964. It is known(e.g. U.S. Pat. No. 4,410,823 Miller et al.) that polarity reversalsresult in partial acoustic wave reflection. The reflection into beam 973and the transmission into beam 961 is affected by the spacing of theinterfaces between the poled regions. If high reflection and lowtransmission is desired, adjacent interfaces should be spaced by adistance equal to an integral multiple of half an acoustic wavelength.If high transmission is required through a structure, with lowreflection, the spacing should be equal to a quarter of an acousticwavelength plus any integral multiple of half a wavelength. By applyingan appropriate number of poled regions near an interface where theacoustic impedance changes, an antireflection (AR) structure can befabricated provided that the phases of the reflected waves are chosen tobe out of phase with and the same amplitude as the reflected wave fromthe interface.

FIG. 44 shows a poled bulk acoustic transducer 971. An input acousticbeam 972 is incident on a poled region of a piezoelectric substrate 965containing a pair of electrodes 974 and 975. The poled region containstwo types of domains 963 and 964 which are optimally reversed domains.The electric field induced by the acoustic wave in each of the poledregions can be selected to be identical by reversing the polingdirection every half acoustic wavelength. In this case, a singleelectrode may be used to pick up the induced voltage instead of theprior art interdigitated electrodes. The electrodes 974 and 975 are usedto detect the presence of the input wave 972. The output voltage, tappedby conductors 979 and seen in the electronic controller 978, variessinusoidally (for a narrowband input) as a function of time with anamplitude related to the amplitude of the acoustic wave. As discussedabove, if the poled interface spacing is a half wavelength, thestructure also acts as a high reflector, which may not be desirable in agiven implementation. This characteristic may be eliminated by spacingthe interfaces alternately at one quarter wavelength and three quartersof a wavelength as shown in FIG. 44. In this case, the structure is anantireflection coating, eliminating the undesired reflection. Sincealmost the entire acoustic wave penetrates into the poled structure,where its energy can be almost totally absorbed into the detectionelectronics, this structure 971 is an efficient tuned detector ofacoustic energy. The bandwidth of the structure is inversely related tothe number of acoustic periods that fit within the poled structurecovered by the electrodes. The efficiency is related to the acousticpath length under the electrodes. The bandwidth and the efficiency ofthe detector are therefore related, and can be adjusted by changing thesize of the detection region.

The structure 971 can also be used as an acoustic generator, essentiallyby running the process in reverse. A time dependent electrical signal isapplied between the two electrodes at the frequency of the acoustic waveit is desired to excite. The piezoelectric coefficient of the substrateproduces a periodic strain at the frequency of the acoustic wave, and apair of waves are generated, one 961 propagating in the forwarddirection and one 973 in the reverse direction. A high efficiencyunidirectional generator can be made if it is desired to generate only asingle wave, by combining the devices 953 and 971, with 953 beingconfigured as a total reflector for the undesired wave. If the totalreflector is oriented at 90° to the undesired wave and the phase of thereflected wave is chosen to be in phase with the desired wave, the twowaves will emerge in a single direction as essentially a single wave.

A variation of the structure of FIG. 44 is a strain-actuated opticalinteraction device. In this device, the poled regions 964 and 963 areactuated by a strain field, producing a change in the index ofrefraction through the photoelastic effect. Now the structure 975 is astrain-inducing pad which may be deposited onto the substrate material965 at an elevated temperature so that the different coefficients ofthermal expansion of the film and the substrate create a strain field atroom temperature. The mechanical strain field, working through thephotoelastic tensor, produces index changes in the substrate whichchange from domain to domain, again producing a substrate with patternedindex of refraction which can be used as described elsewhere herein.Electric fields using the electro-optic effect can be combined with thephotoelastic effect provided that the deposition process of theelectrodes do not undesirably affect the desired strain field.

The structure 890 of FIG. 45 is a tuned coherent detector of pairs oflight waves. It is tuned in the sense that it will only sense frequencydifferences between light waves within a certain bandwidth about adesired central "resonant" frequency difference. In the simplest case,the device is configured with equal separations between interdigitatedelectrodes 885 and 886 which form a periodic structure with period Λ. Ata given instant, the two input frequencies present in the input beam 887produce an interference pattern of electric fields within the waveguide888 with a spatial period which depends on the optical frequencydifference and the index of refraction of the substrate 889 at theoptical frequency. At a frequency difference where the spatial period ofthe interference pattern equals the period Λ, the electrode structure ison resonance, and the electrodes will be excited to a potentialdifference due to the induced displacement charge at the top of thewaveguide.

The frequency response characteristic is related to a sinc² functionwith resonant frequency determined by the optical frequency differenceat which two optical waves slip phase by 2π in a poled grating period.The buffer layer 891 is required to minimize the loss to the propagatingoptical waves when the electrode structure is laid down. It does notsubstantially reduce the strength of the induced potential if itsthickness is much smaller than the period Λ. The interference patternhas a low frequency component which oscillates at the frequencydifference between the two light waves. The electronic signal which ispicked up by the electronic controller 978 via leads 979 therefore alsooscillates at the difference frequency. The amplitude of the electronicsignal is large at the resonance difference frequency, and falls off atother difference frequencies according to the bandwidth of the device,which is related to the inverse of the number of beat periods containedwithin the interdigitated electrode structure.

The interdigitated electrodes may alternately be configured withmultiple frequency components so that there are several resonantfrequencies, or so that the bandwidth of the response is modified. Notealso that the device may be sensitive to multiple orders. If theelectrodes are narrow compared to a half period, there will be asignificant response at the odd harmonics of the resonant differencefrequency. By shifting the fingers relative to each other so that thereis asymmetry along the axis of the waveguide, a responsivity can becreated to the even harmonics. This higher order response can only beimproved at the expense of lowering the first order response. It can beminimized by centering the electrodes relative to each other, and byincreasing their width. Finally, the waveguide 888 is not strictlynecessary. It may be omitted, but the detected waves should be broughtvery close to the electrodes to optimize the signal pickup.

FIG. 46 shows a low loss switchable waveguide splitter 780. This devicehas a permanent wye waveguide splitter 774 consisting of an inputwaveguide segment widening into a wye junction and branching into twooutput waveguide segments 775 and 776 which are both optical pathpossibilities for light incident in the input segment. The widths andindex profiles of the input and output segments are preferably equal.The splitter 780 also has a poled structure 778 which has anelectro-optic coefficient within the region of the wye splitter 774. Thepoled region 778 may be a thin layer near the top of the substrate,which may have multiple layers, or it may extend throughout thesubstrate. The remainder of the substrate may be poled or unpoled. Apair of planar electrodes 777 and 779 are disposed adjacent to eachother over the waveguides, with one electrode 777 covering a portion ofone output waveguide 775, and the other electrode 779 covering a portionof the other output waveguide 776. The electrodes are planar only to theextent that this optimizes fabrication convenience and function: if thesurface they are applied to is flat or curved, they conform. The edge781 of the electrode 777 crosses the waveguide 775 at a very shallowangle, and forms a smooth continuation of the inside edge of thewaveguide 776 at the wye junction. Likewise, the edge 783 of theelectrode 779 crosses the waveguide 776 at a very shallow angle, andforms a smooth continuation of the inside edge of the waveguide 775 atthe wye junction. When the electrodes are excited relative to each otherwith one polarity, the index of refraction under the electrode 777 isdepressed and the index under the electrode 779 is increased. As aresult, an excited region under the electrode edge 781 forms a waveguideboundary, steering the input beam 789 almost entirely into the outputbeam 784 with very little power leakage into the alternate output beam782. The increased index under the electrode 779 aids in steering theoptical energy away from the boundary 781. When the opposite polarity isapplied between the electrodes, the input beam is steered almostentirely into the other output beam 782. If no voltage is applied, theinput power is evenly divided into the two output ports if the structureis made symmetric. This structure is therefore a 3 dB splitter which canbe electrically switched as a beam director into one of two directionswith low loss.

The electrodes 777 and 779 are tapered away from the wye structure 774at the input to the structure forming a gradual approach of the lowerindex region towards the waveguide to minimize optical losses. Thesmoothing effect of the electrostatic field distribution produces a verysmooth index of refraction transition under both electrodes. The edge ofthe electrodes which crosses the output waveguides far from the wyebranching region is preferably arranged at 90° to the waveguide tominimize losses.

The wye splitter may be arranged in an asymmetric way to produce asplitting ratio different from 3 dB with the fields off. This can bedone by increasing the deviation angle for one of the waveguides and/ordecreasing the angle for the other. The switching function operatesalmost as well with an asymmetric structure as with a symmetricstructure, provided that a sufficiently large electric field is appliedwith the electrodes. The extinction ratio (the ratio between the powerin the switched-on waveguide and the power in the switched-offwaveguide) can remain very large despite a large asymmetry. However, theoptical losses will be somewhat different in the two legs of anasymmetric switchable waveguide splitter. The device 780 may, therefore,be configured as a splitter with any desired splitting ratio, and stillbe switched with good efficiency and high extinction ratio.

This device may be cascaded to allow switching among more than twooutput waveguides. If, for instance, the output waveguide 775 isconnected to the input of a second device similar to 780, its power maybe passively or actively switched into an additional pair of waveguides.Sixteen switched output lines may be accomplished with four sets of one,two, four, and eight switches similar to 780. The power division ratioamong these lines may be configured to be equal in the unswitched state,or any other power division ratio. When the switches are activated, asingle output waveguide may be turned on, a single output waveguide maybe turned off, or any combination of output waveguides may be turned onand off.

The direction of propagation of the light in the device may be reversed.In this case, an input on either one of the output ports 775 and 776 canbe switched to emerge from the input port. In the absence of an appliedvoltage, the power at each output port is coupled into the input portwith a given attenuation (3 dB in the case of a symmetric device). Whenthe field is switched on, power in the "on" waveguide is connected intothe input port with very low loss, while the power in the "off"waveguide is very effectively diffracted away from the input waveguide.The "off" waveguide is essentially isolated from the input port.

Alternatively, a mirror image device may be connected back-to-back withthe switch 780 so that the input waveguides join together, forming a 2×2switch or router. An input on either pair of waveguide ports may beswitched into either waveguide of the other port pair. Again, cascadingis possible, producing an n×n switch/router.

FIG. 47 shows an alternative realization 790 of a switchable waveguidesplitter using multiple poled regions. In this configuration, theswitched index difference along the boundaries of the waveguides in thewye region is enhanced, thereby confining better the optical mode into anarrower region, and reducing the residual coupling into theswitched-off output waveguide. Two poled regions 785 and 786 aredisposed on each side of the input waveguide 774 along the wye splittingregion. The poled regions have boundaries 787 and 788 which cross theoutput waveguides 775 and 776 at a very shallow angle, and which form asmooth continuation of the inside edges of the waveguides 776 and 775 atthe wye junction. The boundaries of the poled regions taper slowly awayfrom the input waveguide to allow a slow onset of the electricallyexcited index change, and they cross the output waveguides at a largedistance from the wye junction where the electric field is substantiallyreduced, in order to reduce the optical loss. Electrodes 791 and 792 aredisposed substantially over the poled regions 785 and 786.

A potential difference is applied to the electrodes, exciting anelectric field in an electrostatic pattern throughout the volume betweenand around them. The electric field penetrates the poled regions and thesurrounding regions, inducing a corresponding pattern of optical indexchanges. The local optical index change is related to the product of thelocal electric field direction and the local electro-optic coefficient.The poled regions are preferably surrounded by regions of oppositepolarity, in which case their electro-optic coefficient is of oppositesign to that of the surrounding regions. At the interfaces 787 and 788there is a sharp change in the index of refraction. On one side of thewaveguide, the index is reduced at the interface, producing a guidingtendency away from the low index region. The opposite is true of theother side. If the applied electric field is large enough, the interfacewith the reduced index forms a waveguide boundary. Since the guidinginterface connects smoothly as an extension of the inside boundary ofthe output waveguide across from the poled region, the input light beam789 is guided into that output waveguide. The light leak is low into theswitched-off waveguide if the curvature of the guiding boundary isgradual. There is low loss at the input, because the poled regionsapproach the waveguide slowly. There is low loss at the wye junction,because the portions of the poled regions which extend beyond thejunction depress the guiding effect of the switched-off outputwaveguide, and enhance the guiding of the switched-on output waveguide.

As an alternative, the poled regions could be surrounded by unpoledmaterial. There is still an abrupt change in the index at the interfaces787 and 788 so the device still functions, but the index change is onlyhalf the value obtained when the poled regions are surrounded withreverse poled material, so the applied field must be higher. Thealternatives described before also apply to this device.

FIG. 48 shows the key design elements of a 1×3 switch. The designelements illustrated here show how to transform the device 780 of FIG.46 into a 1×3 switch with a single poled region and patternedelectrodes. The device contains a permanent branching waveguide with thedesired number n (n=three) of output branches. The waveguide passesthrough a poled region which extends deeper than the waveguides (forgood extinction ratio) and significantly beyond the junction regionwhere the waveguides have become separated by a large amount (such asthree times their width). Several zones are defined by the waveguideboundaries, by their smooth extensions back into the boundaries of theinput waveguide, and by normal boundaries across the output waveguidesat a distance significantly beyond the junction region. There are (n²+2n-2)/2 zones so defined. It is useful to extend the outermost zonebeyond the outside of the outermost waveguide as shown to taper theinput. A separate electrode is placed over each of the regions with asmall gap between all electrodes, but sufficient gap to avoid electricalbreakdown when excited.

To operate the device, electric fields are independently applied to thezones with polarity determined by whether or not the corresponding zoneis confined within the desired waveguide. For example, the five zones ofFIG. 48 are excited according to Table I. As before, the magnitude ofthe electric field is adjusted to produce a good guiding boundary alongthe edges of adjacent zones excited at different polarities.

                  TABLE I                                                         ______________________________________                                        Electrode Number                                                                            Top        Middle  Bottom                                       ______________________________________                                        1             +          -       -                                            2             +          +       -                                            3             -          +       -                                            4             -          +       +                                            5             -          -       +                                            ______________________________________                                    

Alternatively, the design elements of FIG. 48 also show how to transformthe device 790 of FIG. 47 into a 1×3 switch with multiple poled regions.The device again contains a permanent branching waveguide with thedesired number n (n=three) of output branches. Again, several zones aredefined by the waveguide boundaries, by their smooth extensions backinto the boundaries of the input waveguide, and by boundaries whichcross the output waveguides at a distance significantly beyond thejunction region. Again, it is useful to extend the outermost zone beyondthe outside of the outermost waveguide as shown, in order to taper theinput. Each zone is poled in the opposite direction to neighboring zoneswith a common zone boundary. Zones with the same poling direction mayshare at most a vertex. Preferably, the input waveguide region is poledoppositely to the innermost zones (i.e. the zones closest to the inputwaveguide). In FIG. 48 the innermost zones are labelled zones 2 and 4.This zone-based polarity selection procedure results in only zones 2 and4 being reverse poled, while zones 1, 3, and 5, which are the outputwaveguide zones, are poled positive (in the same direction as thesurrounding region, if the surrounding region is poled). If four outputwaveguides are used, there are nine zones, six of which are reversepoled, including all of the output waveguide zones. The splitterimplementations which have an even number of output waveguides,therefore, have some advantage because only the even splitters havetheir output waveguide zones poled opposite to a potential substratepoling, with the attendant advantage of increased confinement at thefinal division point and higher transmission for the "on" states andbetter reverse isolation in the "off" states. A separate electrode isplaced over each of the regions.

To operate the device, electric fields are independently applied to thezones, but now the role for the polarity is different. The polarity isdetermined by two factors: whether or not the corresponding zone iscontained within the desired waveguide, and the polarity of the poledregion underneath. For example, if a positive polarity applied to apositively poled region produces an increase in the index of refraction,the following selection rules are followed: if a zone is poled positive,the electrical excitation polarity is selected to be positive if thezone is inside the desired waveguide and negative if the zone isoutside; if a zone is reverse poled (negative), the polarity is selectedto be negative if the zone is inside the desired waveguide, and positiveif the zone is outside. In Table II are shown the optimal polingdirection of the zones for the n=3 case with three output ports as shownin FIG. 48. The design of 1×n and n×n switches is derived by inductionfrom the descriptions of the FIGS. 46, 47 and 48.

                  TABLE II                                                        ______________________________________                                        Zone    Poling Direction                                                                            Top      Middle                                                                              Bottom                                   ______________________________________                                        1       -             -        +     +                                        2       +             +        +     -                                        3       -             +        -     +                                        4       +             -        +     +                                        5       -             +        +     -                                        ______________________________________                                    

The planar components described herein may be stacked into multiplelayer three dimensional structures containing electro-opticallycontrolled devices and waveguide components. Stacks or three-dimensionalconstructions of planar waveguides and switches are fabricated byalternately layering or depositing electro-optically active, polablethin films, preferably polymers, and buffer isolation layers, which maybe either insulating or electrically conducting. Advantages of stackedstructures include better crosstalk isolation due to more widely spacedwaveguide elements. Higher power handling capability is also achievedbecause more optical power can be distributed among the layers.Individual layers can be used if desired to distribute individualwavelengths in a display device.

Once deposited on a suitable substrate, poling of the active opticalwaveguide/switching layer is done using the techniques heretoforedescribed. A buffer layer of lower index is necessary to isolate oneactive layer from adjacent layers, and is designed to establish thedesired guiding in the dimension normal to the plane. Buffer layers ofSiO₂, for example, may be used. Next comes a ground plane which can befabricated from a metallic layer since it is isolated from the opticallyactive layers, followed by a thick buffer layer. The buffer layers mustalso be capable of withstanding the applied voltages between thedifferent layers of electrodes and ground planes. In polymers, a largearea may be poled, and desired regions selectively de-poled by UVirradiation techniques as previously described in order to createwaveguide features, even after a transparent buffer layer, such as SiO₂has been applied. Or, poling can be performed electrically. Withpolymers, de-poling one layer by UV irradiation will not affect thelayer behind it because of the shielding provided by the underlyingmetallic ground plane. Metal electrodes and conductive paths may then belaid down by standard masking and coating techniques, followed byanother insulating buffer layer, and the next active layer. The bufferlayer should be planarized to minimize the loss in the subsequent activeoptical waveguide/switching layer. This process of adding layers may berepeated as often as desired for a given device.

A variation in fabrication technique for making activation paths andelectrodes for the poled device stacks is to coat the electro-opticlayer with an insulating layer that is subsequently doped or infused toproduce electrically conductive patterns within the buffer layer usingstandard lithographic masking techniques. Incorporating the electrodesinto the buffer layer would serve to minimize the thickness of thestacked device.

Hybridized devices consisting of different electro-optically activematerials could be used to ameliorate fabrication complexities. Forexample, the first electro-optically active layer containing waveguidedevices could be fabricated in a LiNbO₃ substrate, which would alsoserve as the support substrate. Next a buffer layer and a layer ofelectrodes for the lithium niobate devices are deposited. Two insulatingbuffer layers sandwiching a conducting plane would then be coated ontothe device prior to depositing the next active layer which could be apolable polymer. Subsequent layers are built up, poled and patterned asdescribed earlier. The conducting planes in between buffer layers mayserve both as electrodes to permit area poling of each polymer layer andto shield previous layers from the poling process.

Stacked waveguide arrays may be used, for example, as steering devicesfor free space beam manipulation. Electrically activated andindividually addressable waveguide elements stacked closely together,and aligned with a source array form a controllable phased array foremitting optical radiation. The relative phases of the beams can beadjusted by varying the voltages on the poled zones as describedpreviously. By adjusting these phases in a linear ramp, the emittedlight from an array of waveguides can be swept in direction rapidlywithin the plane of the array. A linear array of devices on a plane cantherefore sweep within the plane only. However, when poled waveguidearray planes are vertically integrated into a three dimensional bulkdevice, optical beams emanating from the device may be directed in twodimensions.

An extension of this concept is the mode control of multimode laser bararrays using a stack of waveguide grating reflectors. The waveguidestack is dimensionally matched to butt-couple to a laser diode array. Bycontrolling the phase of the individual elements, the emission modepattern of a multi element laser bar can be controlled. In devices wheresingle mode waveguide confinement is not necessary, multimode or bulkarrays may also be stacked, for example, to increase the power handlingcapacity of a switched poled device.

FIG. 49 illustrates an embodiment of the phased array waveguide stacksection 1630 with only a single column of waveguides illustrated forclarity. Optical radiation 1640 enters the stack 1630 through waveguides1638 which have been fabricated in an electro-optically active thin film1650, such as a polable polymer. Here the input beams 1640 are shownstaggered to represent beams of identical wavelength, but with differentphases. Light travels along the waveguides 1638 in which they encounterpoled regions 1634 within which the index of refraction may be modifiedelectronically using the techniques described herein. Beams 1642represent the output of the phased array after each light wave has beenindividually phase adjusted to produce output component beams that arealigned in phase.

Many other input and output wave scenarios are possible. For instance, asingle mode laser beam with a flat phase wavefront could illuminate anarea of waveguide elements, which would then impose arbitrary phasedelays across the spatial mode of the beam, thereby allowing the beam tobe electronically steered in free space. Directional beam controldevices using this method would be much faster and more compact thancurrent mechanical or A-O devices. Using optical-to-electrical pickupdevices described herein or known in the art, phase differences or thepresence of multiple frequency components may be sensed within orexternal to the stacked device in order to provide instantaneousinformation for a feedback loop.

The device segment 1630 represented here is constructed on a substrate1632, such as SiO₂, by alternately depositing electrodes, buffer layers,and polable material in the following manner. A broad area planarelectrode 1654, composed of an opaque metallic film or transparentconductive material such as indium-tin-oxide, is deposited, and followedby an electrically insulating buffer layer 1652, such as SiO₂, whichalso serves as the lower boundary layer for the waveguide 1638fabricated in the next layer of polable material 1650. On top of thepolable layer 1650, another buffer layer 1652 is added to form an upperwaveguide bound before depositing the patterned electrode 1636 used toactivate the poled structures. Another buffer layer 1652 is then added,this time to electrically insulate the patterned electrode from the nextlayer, another broad area planar electrode 1654. The patterned electrode1636 is separated from one planar electrode only by a thick bufferlayer, and from the other by buffer layers and the polable material.Since it is desired to apply fields across the polable material, theelectrical separation across the polable material should be less thanthe separation across the buffer layer only. The layering sequencebetween broad area electrodes is repeated until the last layer ofpolable material 1650, after which only a buffer layer 1652, patternedelectrode 1636, and optional final insulating layer 1652 need be addedto complete the stack. Electrical leads 1646 and 1648 are brought intocontact with electrodes 1636 and 1654, respectively, through integrationand bonding techniques known to the art, and connected to voltagedistribution control unit 1644.

The voltage control unit 1644 serves a dual purpose: to activate thepoled devices individually, and to isolate each from the electric fieldused to control neighboring layers of active elements. The unit 1644would be in essence a collection of coupled floating power supplies inwhich the voltages between electrodes 1636 and 1654 sandwiching anactive layer may be controlled without changing the voltage differencesacross any other active layer.

Region 1634 depicts a poled region with one or more domains, andelectrode 1636 depicts an unbroken or a segmented or patterned regionwith one or more isolated elements. Waveguide stack 1630 is described asa device for phase control, but stacks of waveguide structures mayinclude any number of combinations of poled devices described herein, inseries optically, or otherwise configured.

FIG. 50 shows a prior art adjustable attenuator 1400. An input waveguide1402 traverses an electro-optically active region of a substrate 1404.An input optical beam 1406 propagates along the input waveguide into anoutput waveguide 1408, forming the output optical beam 1410. Electrodes1412, 1414, and 1416 are disposed over the waveguide so that whenelectrode 1414 is excited at a given polarity (positive or negative)with respect to the two electrodes 1412 and 1416, there is an inducedchange in the index of refraction within the segment 1418 region of thewaveguide under and adjacent to the electrodes due to the electro-opticeffect. The electrode configuration is somewhat arbitrary and may bedifferent and more complex than shown in the prior art represented byFIG. 50, but the common factor which all the patterns have in common isthat overall, they reduce the index of refraction in the core whenexcited to a voltage, and increase the index of the surrounding regions.

In the absence of applied electric field, the loss of the waveguidesegments is low, determined primarily by scattering on roughness alongthe waveguide walls. However, when the electric field is applied, theloss can be increased to a very large value. The three electrode patternallows a negative index change within the waveguide at the same time asa positive index change occurs outside the waveguide, substantiallyflattening and broadening the index profile. When the field is applied,the modified section of the waveguide 1418 under the electrodes has amuch wider lowest order mode profile from the input 1402 and output 1408sections of the waveguide. As a result, mode coupling loss occurs bothwhen the input beam 1416 transitions into the section 1418 and when thelight in section 1418 couples back into the output waveguide 1408. Ifthe index changes are large enough, the lowest order mode goes belowcutoff, and the light emerging from the end of the waveguide 1402diffracts almost freely into the substrate, resulting in a largecoupling loss at the beginning of the waveguide 1408.

When a given mode enters the modified section 1418 of the waveguide, theoverlap between its intensity profile and any mode profile of themodified section 1418 is reduced by the change in the index profile ofthe modified segment. If the segment 1418 is multimode, severalpropagating modes and radiation modes will be excited. If it is singlemode, many radiation modes will be excited. The combination of thesemodes then propagates to the far end of the segment 1418 and couplesinto the output waveguide section 1408, where only a fraction of thelight couples back into a mode of the waveguide to form the output beam1410. By controlling the voltage applied to the electrodes, the loss inthe device 1400 can be adjusted from very low to very high.

The maximum loss which can be obtained depends on the magnitude of theindex change, the size of the excited regions, their length, and onwhether the input and output waveguides are single mode or multimode. Ina variation of the geometry, only two electrodes might be disposed overthe waveguide segment 1418, decreasing the index within the waveguidesegment and increasing the index to one side instead of on both sides.The function is again as an attenuator, but the rejected radiationfields will tend to leave the device towards the side of the increasedindex. This ability to direct the lost radiation might be of advantagein some systems where control of the rejected light is desired. Anabsorber may also be placed downstream of the segment 1418, on one orboth sides, to prevent the rejected light from interfering with otherfunctions elsewhere in the system.

FIG. 51 shows a poled switched attenuator 1420. This device is animprovement on the prior art device of FIG. 50 in that poled regions areused to increase the definition of the index change and increase theindex discontinuity, thereby increasing the amount of attenuation whichcan be obtained in a single stage. Regions 1422 and 1424 areelectro-optically poled in a reverse direction from the surroundingmaterial. (As an alternative, the surrounding material may be unpoled,or have no electro-optic coefficient, or it may simply be poleddifferently from the regions 1422 and 1424.) The central electrode 1426covers both poled regions and surrounding material. It is excitedrelative to the electrodes 1428 and 1430 to produce a change in index ofrefraction in the poled regions 1422, 1424, and the surroundingmaterial. The device 1420 operates in a similar way as described abovein reference to the device 1400. The applied voltage reduces andbroadens the index profile of the waveguide segment 1418, reducing thecoupling between the mode of the output waveguide 1408 and the modesexcited in the segment 1418 by the input beam 1406. In thisconfiguration, the change in the index profile is abrupt at thebeginning of the modified waveguide region 1418, and therefore the lossis larger. The number and shape of the poled segments 1422 and 1424 canbe varied so long as the mode coupling with the excited waveguidesegment 1418 is different from the mode coupling with the unexcitedsegment. The device may be configured with high loss in the electricallyunexcited condition, adjusting to low loss in the electrically excitedcondition. In this case the electrically excited regions and/or thepoled regions form a portion of the structure of the waveguide segment1418. The waveguide segment 1418 may itself may be configured in manydifferent ways, most notably if it is absent entirely withoutexcitation, in which case the device is similar to the switchedwaveguide modulator of FIG. 29A.

As described above, these devices may be cascaded, in this case toincrease the maximum attenuation.

The devices of FIG. 50 and FIG. 51 can also be operated as a variableintensity localized ("point") light source. The light propagating inwaveguide 1402 is confined to follow the path of the waveguide until avoltage is applied the electrode structure. When the waveguiding effectis reduced or destroyed by changing the index of refraction, part or allof the previously confined light beam will now propagate according tofree-space diffraction theory. The diffracting beam will continue topropagate in the forward direction while the beam area expands in twodimensions to be much larger than the core of the waveguide 1408. At anappropriate distance away from the electrode structure, the beam areacan fill a large fraction of the substrate aperture and appear to aviewer as a point source of light emanating from a spatial location nearthe electrode structure.

If desired, a one-dimensional localized source can also be constructedwith this technique. The waveguide segment 1418 in FIGS. 50 and 51 canbe embedded in a planar waveguide structure fabricated using techniquesknown to the art, such that when an appropriate voltage level is appliedto the electrode structure, the transverse confinement of the mode isdestroyed while the vertical confinement in the planar waveguide is not.Thus the beam area would expand in one dimension, confining the light toa narrow plane.

FIG. 52 shows a poled device 1500 with an angle broadened poled grating.The method shown for broadening the bandwidth is an alternative to thebandwidth modifying approaches described in reference to FIG. 18 andelsewhere herein. A periodic structure 1500 is shown with poled regions1502 which are preferably reverse poled into a poled region of thesubstrate 1504. Other structures such as waveguides and electrodes andadditional gratings are incorporated as desired. The domain 1502 crossthe central axis of propagation of the input beam 1508 with a patternwhich may be strictly periodic with a 50% duty cycle. The sides of thetop surfaces of the poled regions all align along lines drawn from analignment point 1506. The poled regions approximately reproduce theirsurface shape some distance into the material. The result is a poledstructure with periodicity which changes linearly with the transverseposition in the poled substrate. An input beam 1508 which traverses thepoled region may be a freely propagating Gaussian beam (if the domainsare deeply poled) or it may be confined in a waveguide 1512. Accordingto the function of the grating, the input beam may be coupled into afiltered or frequency converted output beam 1510, or into aretroreflected beam 1514. The range of periodicities in the gratingstructure (and hence its bandwidth) depends on the width of the beam andseparation of the point 1506 from the axis of the beam. By adjustingthese quantities, the bandwidth of the poled structure may be increasedsubstantially over the minimum value determined by the number of firstorder periods which fit in the grating. There is a limit on the maximumdesirable angle for the poled boundaries, and therefore the structureshown in FIG. 52 cannot be extended without limit. However, a longinteraction region can be obtained by cascading several segmentstogether. To maximize the coherence between the segments, theperiodicity of the domains along the central axis of the beam should beunmodified at the joins between segments. There will be at least onewedge shaped domain between segments.

Although increasing the bandwidth of the grating decreases theinteraction strength, it makes a device using that grating significantlyless sensitive to small frequency drifts. For example, a frequencydoubler device using an angle broadened grating is more tolerant oftemperature drifts. Another example application is the channel droppingfilter which tends to have narrow bandwidth because of the stronggratings which must be used. Use of an angle broadened grating enables awidened pass band to accept high bandwidth communications signals. Theangle broadened grating can also be applied in the other gratingconfigurations discussed above.

There are alternatives for implementing the angle broadened gratingwhich do not follow the exact pattern described above. For example, therelationship between the angle of the grating periods and the distancealong the propagation axis might be more complex than linear. Aquadratic or exponential variation might be more appropriate for someapplications where the majority of the interacting power exists at oneend of the grating. The angle broadening technique is also applicable toprior art types of gratings such as indiffused, ion exchanged, andetched gratings.

An alternative angle broadened device 1520 using a curved waveguide isshown in FIG. 53. In this case, the poled regions 1522 have parallelfaces, and the angle of the faces are inclined only relative to thelocal direction of propagation within the guide. Again, the bandwidth isbroadened by the different components of the wave experiencing differentFourier components of the grating. The curved waveguide has a higherloss than the straight waveguide, but large curvatures are not required.Several sections as shown in FIG. 53 may be concatenated, forming forexample a sinuous waveguide structure that waves back and forth aroundan essentially straight line.

FIG. 54 shows a controllable poled lens 1530. Concentrically arrangeddomains 1532, 1534, 1536, and 1538 are poled into an electro-opticsubstrate 1540 with a reverse polarity from that of the substrate.Transparent electrodes 1542 and 1544 are applied to the two opposingsurfaces of the device above and below the poled regions. When anelectric field is applied between the two electrodes, the poled regionshave their index of refraction either increased or decreased accordingto the polarity. The geometry of the poled regions is determined by thediffractive requirements of focussing an optical wave of a given color.The separations between the boundaries varies roughly quadratically withradius. If the application requires focussing a plane wave to a roundspot, for example, the poled regions will be round (for equal focussingin both planes), and separated by decreasing amounts as the diameter ofthe poled region increases. The boundaries of the poled regions aredetermined by the phase of a the desired outgoing wave relative to theincoming wave at the surface of the lens structure. A poled regionboundary occurs every time the relative phase of the waves changes by π.For example, if the incoming wave is a plane wave its phase is constantalong the surface. If the outgoing wave is a converging wave which willfocus at a spot far from the surface, it is essentially a spherical waveand the phase changes in that spherical wave determine the boundaries.The lens 1530 is a phase plate with adjustable phase delay according tothe applied voltage, and the domains occupy the Fresnel zones of theobject.

To focus a plane wave of a given color, a voltage is applied which issufficient to phase retard (or advance) the plane wave by π. Eachdifferent frequency has a different focal length defined by the Fresnelzone structure of the poled lens 1530. Higher frequencies have longerfocal lengths. If it were not for dispersion, every wavelength wouldoptimally focus at the same voltage. The voltage can be adjusted tocompensate for the dispersion in the substrate material 1540. If thevoltage is adjusted away from the optimal value, the amount of lightwhich is focussed to the spot is reduced because the phase of the lightfrom the different zones no longer add optimally. They will interferepartially destructively, reducing the net intensity.

The invention has now been explained with reference to specificembodiments. Other embodiments will be apparent to those of ordinaryskill in the art. Therefore, it is not intended that the invention belimited, except as indicated by the appended claims, which form a partof this invention description.

What is claimed is:
 1. An electrically-controlled frequency-selectivebeam coupler comprising:a solid material, said material having a patternof differing domains, at least a first type of said domains being apoled structure and forming at least two elements alternating with asecond type of said domains; at least a first electrically-conductivematerial forming a first electrode, said first electrode confrontingsaid solid material and bridging at least two of said elements of saidfirst type of poled structure; a first waveguide segment traversing saidsolid material; a second waveguide segment traversing said solidmaterial and intersecting with said first waveguide segment at anintersection with said at least two elements, said at least two elementsbeing disposed transverse of said first waveguide segment and saidsecond waveguide segment, said at least two elements defining a grating.2. The device according to claim 1 wherein said grating is disposed atan angle to said first waveguide segment to reflect optical energy fromsaid first waveguide segment into said second waveguide segment.
 3. Anelectrically-controlled frequency-selective beam coupler comprising:asolid material, said material having a pattern of differing domains, atleast a first type of said domains being a poled structure and formingat least two elements alternating with a second type of said domains; atleast a first electrically-conductive material forming a firstelectrode, said first electrode confronting said solid material andbridging at least two of said elements of said first type of poledstructure; a first waveguide segment traversing said solid material; anda second waveguide segment traversing said solid element and adjacentand nonintersecting with said first waveguide segment, said at least twoelements being disposed transverse of at least one of said firstwaveguide segment and said second waveguide segment, said at least twoelements defining a grating.
 4. An electrically-controlledfrequency-selective beam coupler comprising:a solid material, saidmaterial having a pattern of differing domains, at least a first type ofsaid domains being a poled structure and forming at least two elementsalternating with a second type of said domains and defining a grating;at least a first electrically-conductive material forming a firstelectrode, said first electrode confronting said solid material andbridging at least two of said elements of said first type of poledstructure; a waveguide segment traversing said solid material; awaveguide wye junction having a first arm and a second arm, said wyejunction being coupled to said waveguide segment, said device forcontrolling coupling of optical energy in said waveguide segment withsaid first arm and said second arm through said grating, wherein theelectric field controls power in the first optical mode and the secondoptical mode such that superposition of said first optical mode and saidsecond optical mode result in selective spatial concentration of powerin said waveguide segment.
 5. The device according to claim 4 whereinsaid first optical mode is the lowest order transverse mode of saidfirst waveguide segment, and wherein said electric field controls outputpower splitting between said first arm and said second arm.
 6. Thedevice according to claim 4 wherein said first arm is an input for saidfirst optical mode, and wherein the electric field is used for controloptimization of output coupling into the lowest order transverse mode ofsaid waveguide segment.
 7. An optical wave distributor comprising:afirst optical waveguide segment; a second optical waveguide segment; atleast a third optical waveguide segment; a switching region at ajunction of said first optical waveguide segment, said second opticalwaveguide segment and said third optical waveguide segment, saidswitching region comprising: an electro-optic material; and a pluralityof spatially-distributed planar electrodes disposed adjacent saidelectro-optic material, said planar electrodes covering selected sectorsof said switching region in a pattern, said pattern defining a pluralityof optical path possibilities in said electro-optic material among saidfirst optical waveguide segment, said second optical waveguide segmentand said third optical waveguide segment, said electrodes beingselectively activatable to change index of refraction along selectedoptical path possibilities to define an optical path across saidswitching region selected from said optical path possibilities.
 8. Theoptical wave distributor according to claim 7 further including a commonelectrode disposed opposing said planar electrodes across saidelectro-optic material.
 9. The optical wave distributor according toclaim 7 wherein said first optical waveguide segment, said secondoptical waveguide segment and said third optical waveguide segment, andsaid switching region comprise a single solid material.
 10. The opticalwave distributor according to claim 7 wherein said electrodes areelectrically polarized to cause index of refraction to increase withinsectors defining a selected optical path.
 11. The optical wavedistributor according to claim 7 wherein said electrodes areelectrically polarized to cause index of refraction to decrease withinsectors defining boundaries adjacent to a selected optical path.
 12. Theoptical wave distributor according to claim 7 wherein said sectors areformed of poled material, adjacent sectors being formed of domains ofalternating polarity.
 13. The optical wave distributor according toclaim 12 wherein said electrodes are electrically polarized to causeindex of refraction to increase within sectors defining a selectedoptical path.
 14. The optical wave distributor according to claim 12wherein said electrodes are electrically polarized to cause index ofrefraction to decrease within sectors defining boundaries adjacent to aselected optical path.
 15. An optical wave distributor comprising:afirst optical waveguide segment; a second optical waveguide segment; atleast a third optical waveguide segment; a switching region at ajunction of said first optical waveguide segment, said second opticalwaveguide segment and said third optical waveguide segment, saidswitching region comprising: an electro-optic material; and a pluralityof spatially-distributed planar electrodes disposed adjacent saidelectro-optic material, said planar electrodes covering selected sectorsof said switching region in a pattern, said pattern defining a pluralityof optical path possibilities in said electro-optic material among saidfirst optical waveguide segment, said second optical waveguide segmentand said third optical waveguide segment, said electrodes beingselectively activatable to change index of refraction along selectedoptical path possibilities to define a plurality of optical pathssimultaneously across said switching region selected from said opticalpath possibilities.
 16. An optical wave attenuator comprising:a firstoptical waveguide segment; a second optical waveguide segment; aswitching region at a junction of said first optical waveguide segmentand said second optical waveguide segment, said switching regioncomprising: an electro-optic material; and a plurality ofspatially-distributed electrodes disposed adjacent said electro-opticmaterial, said electrodes covering selected sectors of said switchingregion in a pattern, said pattern defining at least one energy leakageoutlet between said first waveguide segment and said second waveguidesegment in said electro-optic material, said electrodes beingselectively activatable to change index of refraction along said opticalpath across said switching region in order to allow incremental couplingbetween said first waveguide segment and said second waveguide segment.17. The device according to claim 12 wherein said at least two elementsform a grating.
 18. The device according to claim 17 further including asecond grating disposed along one of said first and second waveguidesegments transverse to and overlapping at least evanescentelectromagnetic fields of optical energy of said one of said waveguidesegments.
 19. An electrically-controlled frequency-selective beamcoupler for separating optical energy at different wavelengthscomprising:a solid dielectric material, said material having a patternof differing domains, at least a first type of said domains being apoled structure and forming at least two elements alternating with asecond type of said domains; at least a first electrically-conductivematerial forming a first electrode, said first electrode confrontingsaid solid material and bridging at least two of said elements of saidfirst type of poled structure; a first waveguide segment traversing saidsolid dielectric material; at least a second waveguide segmenttraversing said solid dielectric material and being disposed in closeproximity and substantially parallel to said first waveguide segment atat least a first location and at a second location along the length ofsaid first waveguide segment to permit optical coupling between saidfirst waveguide segment and said second waveguide segment; wherein saidat least two elements form at least a first grating of a first periodand a second grating of a second period, said first grating beingdisposed transverse of said first waveguide segment and said secondwaveguide segment and overlapping at least evanescent electromagneticfields of optical energy in at least one of said first waveguide segmentand said second waveguide segment at said first location, said secondgrating being disposed transverse of said first waveguide segment andsaid second waveguide segment and overlapping at least evanescentelectromagnetic fields of optical energy in at least one of said firstwaveguide segment and said second waveguide segment at said secondlocation; electric field creating means comprising a first electrodemeans for controlling said optical coupling between said first waveguidesegment and said second waveguide segment at said first location and asecond electrode means for controlling said optical coupling betweensaid first waveguide segment and said second waveguide segment at saidsecond location; and control means for controlling selective applicationof electric fields through said first electrode means and said secondelectrode means.
 20. An electrically-controlled frequency-selective beamcoupler comprising:a solid dielectric material, said material having apattern of differing domains, at least a first type of said domainsbeing a poled structure and forming at least two elements alternatingwith a second type of said domains; at least a firstelectrically-conductive material forming a first electrode, said firstelectrode confronting said solid material and bridging at least two ofsaid elements of said first type of poled structure; a first waveguidesegment traversing said solid dielectric material; a second waveguidesegment traversing said solid dielectric material and intersecting withsaid first waveguide segment at an intersection with said at least twoelements, said at least two elements being disposed transverse of saidfirst waveguide segment and said second waveguide segment, said at leasttwo elements forming a grating thereby forming anelectrically-controlled frequency-selective beam coupler, said gratingbeing disposed at an angle to said first waveguide segment to reflectoptical energy from said first waveguide segment into said secondwaveguide segment; a first tapered segment in said first waveguidesegment forming an adiabatic expansion in an input path to said grating;and a second tapered segment in said second waveguide segment forming anadiabatic contraction in an output path from said grating.
 21. Anelectrically-controlled frequency-selective beam coupler comprising:asolid dielectric material, said material having a pattern of differingdomains, at least a first type of said domains being a poled structureand forming at least two elements alternating with a second type of saiddomains; at least a first electrically-conductive material forming afirst electrode, said first electrode confronting said solid materialand bridging at least two of said elements of said first type of poledstructure; wherein said at least two elements form a first grating and asecond grating, and further including: a first waveguide segment alongan optical axis; a second waveguide segment disposed adjacent said firstwaveguide segment in sufficiently close proximity to permit couplingbetween said first waveguide segment and said second waveguide segment;said first grating being disposed adjacent to and transverse of saidfirst waveguide segment; said first electrode being disposed adjacentsaid first grating; said second grating being disposed adjacent to andtransverse of said first waveguide segment; a second electrode disposedadjacent said second grating; electrode means opposing said firstelectrode and said second electrode capable of establishing controlelectric fields through said first grating and said second grating; saidfirst grating having at least a first period along said optical axissubstantially equal to a second period of said second grating; saidfirst grating and said second grating being separated by a predeterminedoptical distance.
 22. An electrically-controlled frequency-selectivebeam coupler comprising:a solid dielectric material, said materialhaving a pattern of differing domains, at least a first type of saiddomains being a poled structure and forming at least two elementsalternating with a second type of said domains; at least a firstelectrically-conductive material forming a first electrode, said firstelectrode confronting said solid material and bridging at least two ofsaid elements of said first type of poled structure; wherein said atleast two elements form a first grating and a second grating;and furtherincluding: a first waveguide segment along an optical axis; a secondwaveguide segment disposed adjacent said first waveguide segment insufficiently close proximity to permit coupling between said firstwaveguide segment and said second waveguide segment; said first gratingbeing disposed adjacent to and transverse of said first waveguidesegment; said first electrode being disposed adjacent said firstgrating; said second grating being disposed adjacent to and transverseof said second waveguide segment; a second electrode disposed adjacentsaid second grating; electrode means opposing said first electrode andsaid second electrode for establishing control electric fields throughsaid first grating and said second grating; said first grating having atleast a first period along said optical axis substantially equal to asecond period of said second grating; said first grating and said secondgrating being separated by a predetermined optical distance; and whereinsaid control electric fields are applied through said first grating andsaid second grating.